diff --git a/app/build.gradle b/app/build.gradle
index d878ad40..0646c92f 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -17,8 +17,8 @@ android {
         minSdkVersion 14
         targetSdkVersion 23
 
-        versionCode 75
-        versionName "0.9-alpha3"
+        versionCode 76
+        versionName "0.9-alpha4"
 
         buildConfigField "java.util.Date", "buildTime", "new java.util.Date()"
     }
@@ -49,8 +49,8 @@ configurations.all {
 }
 
 dependencies {
-    compile 'com.google.guava:guava:18.0'
     compile 'dnsjava:dnsjava:2.1.7'
+    compile 'org.apache.commons:commons-lang3:3.4'
     provided 'org.projectlombok:lombok:1.16.6'
     compile('org.slf4j:slf4j-android:1.7.12')
 
diff --git a/app/src/main/java/at/bitfire/davdroid/Constants.java b/app/src/main/java/at/bitfire/davdroid/Constants.java
index fdc02ef6..a9a33eaf 100644
--- a/app/src/main/java/at/bitfire/davdroid/Constants.java
+++ b/app/src/main/java/at/bitfire/davdroid/Constants.java
@@ -14,8 +14,16 @@ public class Constants {
 	public static final String
 		ACCOUNT_TYPE = "bitfire.at.davdroid",
 		WEB_URL_MAIN = "https://davdroid.bitfire.at/?pk_campaign=davdroid-app",
-		WEB_URL_HELP = "https://davdroid.bitfire.at/configuration?pk_campaign=davdroid-app",
-		WEB_URL_VIEW_LOGS = "https://github.com/bitfireAT/davdroid/wiki/How-to-view-the-logs";
+		WEB_URL_HELP = "https://davdroid.bitfire.at/configuration?pk_campaign=davdroid-app";
 
     public static final Logger log = LoggerFactory.getLogger("davdroid");
+
+    // notification IDs
+    public final static int
+            NOTIFICATION_ANDROID_VERSION_UPDATED = 0,
+            NOTIFICATION_ACCOUNT_SETTINGS_UPDATED = 1,
+            NOTIFICATION_CONTACTS_SYNC = 10,
+            NOTIFICATION_CALENDAR_SYNC = 11,
+            NOTIFICATION_TASK_SYNC = 12;
+
 }
diff --git a/app/src/main/java/at/bitfire/davdroid/resource/LocalAddressBook.java b/app/src/main/java/at/bitfire/davdroid/resource/LocalAddressBook.java
index e0956c2b..8d9ecf9e 100644
--- a/app/src/main/java/at/bitfire/davdroid/resource/LocalAddressBook.java
+++ b/app/src/main/java/at/bitfire/davdroid/resource/LocalAddressBook.java
@@ -26,7 +26,9 @@ import lombok.Synchronized;
 
 public class LocalAddressBook extends AndroidAddressBook implements LocalCollection {
 
-    protected static final String SYNC_STATE_CTAG = "ctag";
+    protected static final String
+            SYNC_STATE_CTAG = "ctag",
+            SYNC_STATE_URL = "url";
 
     private Bundle syncState = new Bundle();
 
@@ -80,6 +82,27 @@ public class LocalAddressBook extends AndroidAddressBook implements LocalCollect
             syncState.clear();
     }
 
+    protected void writeSyncState() throws ContactsStorageException {
+        @Cleanup("recycle") Parcel parcel = Parcel.obtain();
+        parcel.writeBundle(syncState);
+        setSyncState(parcel.marshall());
+    }
+
+    public String getURL() throws ContactsStorageException {
+        synchronized (syncState) {
+            readSyncState();
+            return syncState.getString(SYNC_STATE_URL);
+        }
+    }
+
+    public void setURL(String url) throws ContactsStorageException {
+        synchronized (syncState) {
+            readSyncState();
+            syncState.putString(SYNC_STATE_URL, url);
+            writeSyncState();
+        }
+    }
+
     @Override
     public String getCTag() throws ContactsStorageException {
         synchronized (syncState) {
@@ -93,11 +116,7 @@ public class LocalAddressBook extends AndroidAddressBook implements LocalCollect
         synchronized (syncState) {
             readSyncState();
             syncState.putString(SYNC_STATE_CTAG, cTag);
-
-            // write sync state bundle
-            @Cleanup("recycle") Parcel parcel = Parcel.obtain();
-            parcel.writeBundle(syncState);
-            setSyncState(parcel.marshall());
+            writeSyncState();
         }
     }
 
diff --git a/app/src/main/java/at/bitfire/davdroid/resource/LocalCalendar.java b/app/src/main/java/at/bitfire/davdroid/resource/LocalCalendar.java
index c2bb046c..84972b03 100644
--- a/app/src/main/java/at/bitfire/davdroid/resource/LocalCalendar.java
+++ b/app/src/main/java/at/bitfire/davdroid/resource/LocalCalendar.java
@@ -24,11 +24,10 @@ import android.provider.CalendarContract.Events;
 import android.provider.CalendarContract.Reminders;
 import android.text.TextUtils;
 
-import com.google.common.base.Joiner;
-
-import net.fortuna.ical4j.data.CalendarBuilder;
 import net.fortuna.ical4j.model.component.VTimeZone;
 
+import org.apache.commons.lang3.StringUtils;
+
 import java.io.FileNotFoundException;
 import java.util.LinkedList;
 import java.util.List;
@@ -96,8 +95,8 @@ public class LocalCalendar extends AndroidCalendar implements LocalCollection {
         }
         values.put(Calendars.ALLOWED_REMINDERS, Reminders.METHOD_ALERT);
         if (Build.VERSION.SDK_INT >= 15) {
-            values.put(Calendars.ALLOWED_AVAILABILITY, Joiner.on(",").join(Reminders.AVAILABILITY_TENTATIVE, Reminders.AVAILABILITY_FREE, Reminders.AVAILABILITY_BUSY));
-            values.put(Calendars.ALLOWED_ATTENDEE_TYPES, Joiner.on(",").join(CalendarContract.Attendees.TYPE_OPTIONAL, CalendarContract.Attendees.TYPE_REQUIRED, CalendarContract.Attendees.TYPE_RESOURCE));
+            values.put(Calendars.ALLOWED_AVAILABILITY, StringUtils.join(new int[] { Reminders.AVAILABILITY_TENTATIVE, Reminders.AVAILABILITY_FREE, Reminders.AVAILABILITY_BUSY }, ","));
+            values.put(Calendars.ALLOWED_ATTENDEE_TYPES, StringUtils.join(new int[] { CalendarContract.Attendees.TYPE_OPTIONAL, CalendarContract.Attendees.TYPE_REQUIRED, CalendarContract.Attendees.TYPE_RESOURCE }, ", "));
         }
         return create(account, provider, values);
     }
diff --git a/app/src/main/java/at/bitfire/davdroid/syncadapter/AccountSettings.java b/app/src/main/java/at/bitfire/davdroid/syncadapter/AccountSettings.java
index dd9f70d7..52edc33a 100644
--- a/app/src/main/java/at/bitfire/davdroid/syncadapter/AccountSettings.java
+++ b/app/src/main/java/at/bitfire/davdroid/syncadapter/AccountSettings.java
@@ -9,6 +9,9 @@ package at.bitfire.davdroid.syncadapter;
 
 import android.accounts.Account;
 import android.accounts.AccountManager;
+import android.app.Notification;
+import android.app.NotificationManager;
+import android.content.ContentProviderClient;
 import android.content.ContentResolver;
 import android.content.ContentUris;
 import android.content.ContentValues;
@@ -16,32 +19,34 @@ import android.content.Context;
 import android.content.PeriodicSync;
 import android.database.Cursor;
 import android.net.Uri;
+import android.os.Build;
 import android.os.Bundle;
 import android.provider.CalendarContract;
 import android.provider.CalendarContract.Calendars;
-import android.util.Log;
+import android.provider.ContactsContract;
+import android.text.TextUtils;
+
+import org.apache.commons.lang3.math.NumberUtils;
 
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.List;
 
+import at.bitfire.davdroid.Constants;
+import at.bitfire.davdroid.R;
+import at.bitfire.davdroid.resource.LocalAddressBook;
 import at.bitfire.davdroid.resource.ServerInfo;
-import ezvcard.VCardVersion;
+import at.bitfire.vcard4android.ContactsStorageException;
 import lombok.Cleanup;
 
 public class AccountSettings {
-	private final static String TAG = "davdroid.AccountSettings";
-	
-	private final static int CURRENT_VERSION = 1;
+	private final static int CURRENT_VERSION = 2;
 	private final static String
 		KEY_SETTINGS_VERSION = "version",
-		
+
 		KEY_USERNAME = "user_name",
 		KEY_AUTH_PREEMPTIVE = "auth_preemptive",
-		
-		KEY_ADDRESSBOOK_URL = "addressbook_url",
-		KEY_ADDRESSBOOK_CTAG = "addressbook_ctag",
-		KEY_ADDRESSBOOK_VCARD_VERSION = "addressbook_vcard_version";
+        KEY_LAST_ANDROID_VERSION = "last_android_version";
 
 	public final static long SYNC_INTERVAL_MANUALLY = -1;
 
@@ -50,7 +55,7 @@ public class AccountSettings {
 	final Account account;
 	
 	
-	public AccountSettings(Context context, Account account) {
+    public AccountSettings(Context context, Account account) {
 		this.context = context;
 		this.account = account;
 		
@@ -62,10 +67,43 @@ public class AccountSettings {
 				version = Integer.parseInt(accountManager.getUserData(account, KEY_SETTINGS_VERSION));
 			} catch(NumberFormatException e) {
 			}
-			if (version < CURRENT_VERSION)
-				update(version);
+            Constants.log.info("AccountSettings version: v" + version + ", should be: " + version);
+
+			if (version < CURRENT_VERSION) {
+                showNotification(Constants.NOTIFICATION_ACCOUNT_SETTINGS_UPDATED,
+                        context.getString(R.string.settings_version_update_title),
+                        context.getString(R.string.settings_version_update_description));
+                update(version);
+            }
+
+            // check whether Android version has changed
+            int lastAndroidVersion = NumberUtils.toInt(accountManager.getUserData(account, KEY_LAST_ANDROID_VERSION));
+            if (lastAndroidVersion < Build.VERSION.SDK_INT) {
+                // notify user
+                showNotification(Constants.NOTIFICATION_ANDROID_VERSION_UPDATED,
+                        context.getString(R.string.settings_android_update_title),
+                        context.getString(R.string.settings_android_update_description));
+
+                accountManager.setUserData(account, KEY_LAST_ANDROID_VERSION, String.valueOf(Build.VERSION.SDK_INT));
+            }
 		}
 	}
+
+    protected void showNotification(int id, String title, String message) {
+        NotificationManager nm = (NotificationManager)context.getSystemService(Context.NOTIFICATION_SERVICE);
+        Notification.Builder n = new Notification.Builder(context);
+        if (Build.VERSION.SDK_INT >= 16) {
+            n.setPriority(Notification.PRIORITY_HIGH);
+            n.setStyle(new Notification.BigTextStyle().bigText(message));
+        } if (Build.VERSION.SDK_INT >= 20)
+            n.setLocalOnly(true);
+        if (Build.VERSION.SDK_INT >= 21)
+            n.setCategory(Notification.CATEGORY_SYSTEM);
+        n.setSmallIcon(R.drawable.ic_launcher);
+        n.setContentTitle(title);
+        n.setContentText(message);
+        nm.notify(id, Build.VERSION.SDK_INT >= 16 ? n.build() : n.getNotification());
+    }
 	
 	
 	public static Bundle createBundle(ServerInfo serverInfo) {
@@ -73,11 +111,6 @@ public class AccountSettings {
 		bundle.putString(KEY_SETTINGS_VERSION, String.valueOf(CURRENT_VERSION));
 		bundle.putString(KEY_USERNAME, serverInfo.getUserName());
 		bundle.putString(KEY_AUTH_PREEMPTIVE, Boolean.toString(serverInfo.isAuthPreemptive()));
-		for (ServerInfo.ResourceInfo addressBook : serverInfo.getAddressBooks())
-			if (addressBook.isEnabled()) {
-				bundle.putString(KEY_ADDRESSBOOK_URL, addressBook.getURL());
-				break;
-			}
 		return bundle;
 	}
 	
@@ -123,66 +156,45 @@ public class AccountSettings {
 		}
 	}
 
-
-	// address book (CardDAV) settings
-	
-	public String getAddressBookURL() {
-		return accountManager.getUserData(account, KEY_ADDRESSBOOK_URL);
-	}
-	
-	public String getAddressBookCTag() {
-		return accountManager.getUserData(account, KEY_ADDRESSBOOK_CTAG);
-	}
-	
-	public void setAddressBookCTag(String cTag) {
-		accountManager.setUserData(account, KEY_ADDRESSBOOK_CTAG, cTag);
-	}
-	
-	public VCardVersion getAddressBookVCardVersion() {
-		VCardVersion version = VCardVersion.V3_0;
-		String versionStr = accountManager.getUserData(account, KEY_ADDRESSBOOK_VCARD_VERSION);
-		if (versionStr != null)
-			version = VCardVersion.valueOfByStr(versionStr);
-		return version;
-	}
-
-	public void setAddressBookVCardVersion(VCardVersion version) {
-		accountManager.setUserData(account, KEY_ADDRESSBOOK_VCARD_VERSION, version.getVersion());
-	}
-	
 	
 	// update from previous account settings
 	
 	private void update(int fromVersion) {
-		Log.i(TAG, "Account settings must be updated from v" + fromVersion + " to v" + CURRENT_VERSION);
-		for (int toVersion = CURRENT_VERSION; toVersion > fromVersion; toVersion--)
-			update(fromVersion, toVersion);
+		for (int toVersion = fromVersion + 1; toVersion <= CURRENT_VERSION; toVersion++)
+            updateTo(toVersion);
 	}
 	
-	private void update(int fromVersion, int toVersion) {
-		Log.i(TAG, "Updating account settings from v" + fromVersion + " to " + toVersion);
+	private void updateTo(int toVersion) {
+        final int fromVersion = toVersion - 1;
+        Constants.log.info("Updating account settings from v" + fromVersion + " to " + toVersion);
 		try {
-			if (fromVersion == 0 && toVersion == 1)
-				update_0_1();
-			else
-				Log.wtf(TAG, "Don't know how to update settings from v" + fromVersion + " to v" + toVersion);
+			switch (toVersion) {
+                case 1:
+                    update_0_1();
+                    break;
+                case 2:
+                    update_1_2();
+                    break;
+                default:
+                    Constants.log.error("Don't know how to update settings from v" + fromVersion + " to v" + toVersion);
+            }
 		} catch(Exception e) {
-			Log.e(TAG, "Couldn't update account settings (DAVdroid will probably crash)!", e);
+            Constants.log.error("Couldn't update account settings (DAVdroid will probably crash)!", e);
 		}
 	}
 	
 	private void update_0_1() throws URISyntaxException {
 		String	v0_principalURL = accountManager.getUserData(account, "principal_url"),
 				v0_addressBookPath = accountManager.getUserData(account, "addressbook_path");
-		Log.d(TAG, "Old principal URL = " + v0_principalURL);
-		Log.d(TAG, "Old address book path = " + v0_addressBookPath);
+        Constants.log.debug("Old principal URL = " + v0_principalURL);
+        Constants.log.debug("Old address book path = " + v0_addressBookPath);
 		
 		URI principalURI = new URI(v0_principalURL);
 
 		// update address book
 		if (v0_addressBookPath != null) {
 			String addressBookURL = principalURI.resolve(v0_addressBookPath).toASCIIString();
-			Log.d(TAG, "New address book URL = " + addressBookURL);
+            Constants.log.debug("New address book URL = " + addressBookURL);
 			accountManager.setUserData(account, "addressbook_url", addressBookURL);
 		}
 		
@@ -197,7 +209,7 @@ public class AccountSettings {
 			int id = cursor.getInt(0);
 			String	v0_path = cursor.getString(1),
 					v1_url = principalURI.resolve(v0_path).toASCIIString();
-			Log.d(TAG, "Updating calendar #" + id + " name: " + v0_path + " -> " + v1_url);
+            Constants.log.debug("Updating calendar #" + id + " name: " + v0_path + " -> " + v1_url);
 			Uri calendar = ContentUris.appendId(Calendars.CONTENT_URI.buildUpon()
 					.appendQueryParameter(Calendars.ACCOUNT_NAME, account.name)
 					.appendQueryParameter(Calendars.ACCOUNT_TYPE, account.type)
@@ -205,14 +217,42 @@ public class AccountSettings {
 			ContentValues newValues = new ContentValues(1);
 			newValues.put(Calendars.NAME, v1_url);
 			if (resolver.update(calendar, newValues, null, null) != 1)
-				Log.e(TAG, "Number of modified calendars != 1");
+                Constants.log.debug("Number of modified calendars != 1");
 		}
-		
-		Log.d(TAG, "Cleaning old principal URL and address book path");
+
 		accountManager.setUserData(account, "principal_url", null);
 		accountManager.setUserData(account, "addressbook_path", null);
-		
-		Log.d(TAG, "Updated settings successfully!");
+
 		accountManager.setUserData(account, KEY_SETTINGS_VERSION, "1");
 	}
+
+    private void update_1_2() throws ContactsStorageException {
+        /* - KEY_ADDRESSBOOK_URL ("addressbook_url"),,
+           - KEY_ADDRESSBOOK_CTAG ("addressbook_ctag"),
+           - KEY_ADDRESSBOOK_VCARD_VERSION ("addressbook_vcard_version") are not used anymore (now stored in ContactsContract.SyncState)
+           - KEY_LAST_ANDROID_VERSION ("last_android_version") has been added
+        */
+
+        // move previous address book info to ContactsContract.SyncState
+        @Cleanup("release") ContentProviderClient provider = context.getContentResolver().acquireContentProviderClient(ContactsContract.AUTHORITY);
+        if (provider != null) {
+            LocalAddressBook addr = new LocalAddressBook(account, provider);
+
+            String url = accountManager.getUserData(account, "addressbook_url");
+            if (!TextUtils.isEmpty(url))
+                addr.setURL(url);
+            accountManager.setUserData(account, "addressbook_url", null);
+
+            String cTag = accountManager.getUserData(account, "addressbook_ctag");
+            if (!TextUtils.isEmpty(cTag))
+                addr.setCTag(cTag);
+            accountManager.setUserData(account, "addressbook_ctag", null);
+        }
+
+        // store current Android version
+        accountManager.setUserData(account, KEY_LAST_ANDROID_VERSION, String.valueOf(Build.VERSION.SDK_INT));
+
+        accountManager.setUserData(account, KEY_SETTINGS_VERSION, "2");
+    }
+
 }
diff --git a/app/src/main/java/at/bitfire/davdroid/syncadapter/CalendarSyncManager.java b/app/src/main/java/at/bitfire/davdroid/syncadapter/CalendarSyncManager.java
index f3793b33..a8c60ce5 100644
--- a/app/src/main/java/at/bitfire/davdroid/syncadapter/CalendarSyncManager.java
+++ b/app/src/main/java/at/bitfire/davdroid/syncadapter/CalendarSyncManager.java
@@ -16,13 +16,14 @@ import android.os.Bundle;
 import android.provider.CalendarContract.Calendars;
 import android.text.TextUtils;
 
-import com.google.common.base.Charsets;
-import com.google.common.base.Joiner;
 import com.squareup.okhttp.HttpUrl;
 import com.squareup.okhttp.MediaType;
 import com.squareup.okhttp.RequestBody;
 import com.squareup.okhttp.ResponseBody;
 
+import org.apache.commons.codec.Charsets;
+import org.apache.commons.lang3.StringUtils;
+
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -54,13 +55,11 @@ import lombok.Cleanup;
 
 public class CalendarSyncManager extends SyncManager {
 
-    protected static final int
-            MAX_MULTIGET = 30,
-            NOTIFICATION_ID = 2;
+    protected static final int MAX_MULTIGET = 20;
 
 
     public CalendarSyncManager(Context context, Account account, Bundle extras, SyncResult result, LocalCalendar calendar) {
-        super(NOTIFICATION_ID, context, account, extras, result);
+        super(Constants.NOTIFICATION_CALENDAR_SYNC, context, account, extras, result);
         localCollection = calendar;
     }
 
@@ -126,7 +125,7 @@ public class CalendarSyncManager extends SyncManager {
 
         // download new/updated iCalendars from server
         for (DavResource[] bunch : ArrayUtils.partition(toDownload.toArray(new DavResource[toDownload.size()]), MAX_MULTIGET)) {
-            Constants.log.info("Downloading " + Joiner.on(" + ").join(bunch));
+            Constants.log.info("Downloading " + StringUtils.join(bunch, ", "));
 
             if (bunch.length == 1) {
                 // only one contact, use GET
diff --git a/app/src/main/java/at/bitfire/davdroid/syncadapter/ContactsSyncManager.java b/app/src/main/java/at/bitfire/davdroid/syncadapter/ContactsSyncManager.java
index 99480451..bc81238e 100644
--- a/app/src/main/java/at/bitfire/davdroid/syncadapter/ContactsSyncManager.java
+++ b/app/src/main/java/at/bitfire/davdroid/syncadapter/ContactsSyncManager.java
@@ -13,9 +13,7 @@ import android.content.ContentProviderClient;
 import android.content.Context;
 import android.content.SyncResult;
 import android.os.Bundle;
-import android.text.TextUtils;
 
-import com.google.common.base.Charsets;
 import com.squareup.okhttp.HttpUrl;
 import com.squareup.okhttp.MediaType;
 import com.squareup.okhttp.Request;
@@ -23,6 +21,9 @@ import com.squareup.okhttp.RequestBody;
 import com.squareup.okhttp.Response;
 import com.squareup.okhttp.ResponseBody;
 
+import org.apache.commons.codec.Charsets;
+import org.apache.commons.lang3.StringUtils;
+
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
@@ -30,8 +31,6 @@ import java.nio.charset.Charset;
 import java.util.HashMap;
 import java.util.LinkedList;
 import java.util.List;
-import java.util.Map;
-import java.util.Set;
 
 import at.bitfire.dav4android.DavAddressBook;
 import at.bitfire.dav4android.DavResource;
@@ -56,27 +55,25 @@ import lombok.Cleanup;
 import lombok.RequiredArgsConstructor;
 
 public class ContactsSyncManager extends SyncManager {
-    protected static final int
-            MAX_MULTIGET = 10,
-            NOTIFICATION_ID = 1;
+    protected static final int MAX_MULTIGET = 10;
 
     final protected ContentProviderClient provider;
     protected boolean hasVCard4;
 
 
     public ContactsSyncManager(Context context, Account account, Bundle extras, ContentProviderClient provider, SyncResult result) {
-        super(NOTIFICATION_ID, context, account, extras, result);
+        super(Constants.NOTIFICATION_CONTACTS_SYNC, context, account, extras, result);
         this.provider = provider;
     }
 
 
     @Override
-    protected void prepare() {
-        collectionURL = HttpUrl.parse(settings.getAddressBookURL());
-        davCollection = new DavAddressBook(httpClient, collectionURL);
-
+    protected void prepare() throws ContactsStorageException {
         // prepare local address book
         localCollection = new LocalAddressBook(account, provider);
+
+        collectionURL = HttpUrl.parse(localAddressBook().getURL());
+        davCollection = new DavAddressBook(httpClient, collectionURL);
     }
 
     @Override
@@ -122,7 +119,7 @@ public class ContactsSyncManager extends SyncManager {
 
         // download new/updated VCards from server
         for (DavResource[] bunch : ArrayUtils.partition(toDownload.toArray(new DavResource[toDownload.size()]), MAX_MULTIGET)) {
-            Constants.log.info("Downloading " + TextUtils.join(" + ", bunch));
+            Constants.log.info("Downloading " + StringUtils.join(bunch, ", "));
 
             if (bunch.length == 1) {
                 // only one contact, use GET
diff --git a/app/src/main/java/at/bitfire/davdroid/syncadapter/SyncManager.java b/app/src/main/java/at/bitfire/davdroid/syncadapter/SyncManager.java
index d39ff89a..a5a9c3eb 100644
--- a/app/src/main/java/at/bitfire/davdroid/syncadapter/SyncManager.java
+++ b/app/src/main/java/at/bitfire/davdroid/syncadapter/SyncManager.java
@@ -204,6 +204,9 @@ abstract public class SyncManager {
                     .setContentTitle(context.getString(R.string.sync_error_title, account.name))
                     .setContentIntent(PendingIntent.getActivity(context, 0, detailsIntent, PendingIntent.FLAG_UPDATE_CURRENT));
 
+            if (Build.VERSION.SDK_INT >= 20)
+                builder.setLocalOnly(true);
+
             try {
                 String[] phases = context.getResources().getStringArray(R.array.sync_error_phases);
                 String message = context.getString(messageString, phases[syncPhase]);
@@ -225,7 +228,7 @@ abstract public class SyncManager {
     }
 
 
-    abstract protected void prepare();
+    abstract protected void prepare() throws ContactsStorageException;
 
     abstract protected void queryCapabilities() throws IOException, HttpException, DavException, CalendarStorageException, ContactsStorageException;
 
diff --git a/app/src/main/java/at/bitfire/davdroid/syncadapter/TasksSyncManager.java b/app/src/main/java/at/bitfire/davdroid/syncadapter/TasksSyncManager.java
index b0ab3c67..c10fe465 100644
--- a/app/src/main/java/at/bitfire/davdroid/syncadapter/TasksSyncManager.java
+++ b/app/src/main/java/at/bitfire/davdroid/syncadapter/TasksSyncManager.java
@@ -15,13 +15,13 @@ import android.content.SyncResult;
 import android.os.Bundle;
 import android.text.TextUtils;
 
-import com.google.common.base.Charsets;
-import com.google.common.base.Joiner;
 import com.squareup.okhttp.HttpUrl;
 import com.squareup.okhttp.MediaType;
 import com.squareup.okhttp.RequestBody;
 import com.squareup.okhttp.ResponseBody;
 
+import org.apache.commons.codec.Charsets;
+import org.apache.commons.lang3.StringUtils;
 import org.dmfs.provider.tasks.TaskContract.TaskLists;
 
 import java.io.ByteArrayInputStream;
@@ -56,15 +56,13 @@ import lombok.Cleanup;
 
 public class TasksSyncManager extends SyncManager {
 
-    protected static final int
-            MAX_MULTIGET = 30,
-            NOTIFICATION_ID = 3;
+    protected static final int MAX_MULTIGET = 30;
 
     final protected TaskProvider provider;
 
 
     public TasksSyncManager(Context context, Account account, Bundle extras, TaskProvider provider, SyncResult result, LocalTaskList taskList) {
-        super(NOTIFICATION_ID, context, account, extras, result);
+        super(Constants.NOTIFICATION_TASK_SYNC, context, account, extras, result);
         this.provider = provider;
         localCollection = taskList;
     }
@@ -124,7 +122,7 @@ public class TasksSyncManager extends SyncManager {
 
         // download new/updated iCalendars from server
         for (DavResource[] bunch : ArrayUtils.partition(toDownload.toArray(new DavResource[toDownload.size()]), MAX_MULTIGET)) {
-            Constants.log.info("Downloading " + Joiner.on(" + ").join(bunch));
+            Constants.log.info("Downloading " + StringUtils.join(bunch, ", "));
 
             if (bunch.length == 1) {
                 // only one contact, use GET
diff --git a/app/src/main/java/at/bitfire/davdroid/ui/DebugInfoActivity.java b/app/src/main/java/at/bitfire/davdroid/ui/DebugInfoActivity.java
index c5244828..ef07d2b0 100644
--- a/app/src/main/java/at/bitfire/davdroid/ui/DebugInfoActivity.java
+++ b/app/src/main/java/at/bitfire/davdroid/ui/DebugInfoActivity.java
@@ -23,18 +23,12 @@ import android.view.Menu;
 import android.view.MenuItem;
 import android.widget.TextView;
 
-import com.google.common.base.Throwables;
-
 import org.apache.commons.lang3.exception.ExceptionUtils;
 
-import java.io.PrintWriter;
-import java.io.StringWriter;
-
 import at.bitfire.dav4android.exception.HttpException;
 import at.bitfire.davdroid.BuildConfig;
 import at.bitfire.davdroid.Constants;
 import at.bitfire.davdroid.R;
-import lombok.Cleanup;
 
 public class DebugInfoActivity extends Activity {
     public static final String
@@ -141,7 +135,8 @@ public class DebugInfoActivity extends Activity {
 
         if (exception != null) {
             report.append("STACK TRACE\n");
-            report.append(Throwables.getStackTraceAsString(exception));
+            for (String stackTrace : ExceptionUtils.getRootCauseStackTrace(exception))
+                report.append(stackTrace + "\n");
         }
 
         return report.toString();
diff --git a/app/src/main/java/at/bitfire/davdroid/ui/settings/AccountFragment.java b/app/src/main/java/at/bitfire/davdroid/ui/settings/AccountFragment.java
index 33a72d6a..d3aa45e5 100644
--- a/app/src/main/java/at/bitfire/davdroid/ui/settings/AccountFragment.java
+++ b/app/src/main/java/at/bitfire/davdroid/ui/settings/AccountFragment.java
@@ -141,22 +141,5 @@ public class AccountFragment extends PreferenceFragment {
 			prefSyncTasks.setSummary(R.string.settings_sync_summary_not_available);
 		}
 
-		// category: address book
-		final CheckBoxPreference prefVCard4 = (CheckBoxPreference) findPreference("vcard4_support");
-		if (settings.getAddressBookURL() != null) {     // does this account even have an address book?
-			final VCardVersion vCardVersion = settings.getAddressBookVCardVersion();
-			prefVCard4.setChecked(vCardVersion == VCardVersion.V4_0);
-			prefVCard4.setOnPreferenceChangeListener(new Preference.OnPreferenceChangeListener() {
-				@Override
-				public boolean onPreferenceChange(Preference preference, Object newValue) {
-					// don't change the value (it's not really a setting, only a display)
-					return false;
-				}
-			});
-		} else {
-			// account doesn't have an adress book, disable contact settings
-			prefVCard4.setEnabled(false);
-		}
-
 	}
 }
diff --git a/app/src/main/java/at/bitfire/davdroid/ui/setup/AccountDetailsFragment.java b/app/src/main/java/at/bitfire/davdroid/ui/setup/AccountDetailsFragment.java
index 18d7c8c4..c34da60f 100644
--- a/app/src/main/java/at/bitfire/davdroid/ui/setup/AccountDetailsFragment.java
+++ b/app/src/main/java/at/bitfire/davdroid/ui/setup/AccountDetailsFragment.java
@@ -104,6 +104,11 @@ public class AccountDetailsFragment extends Fragment implements TextWatcher {
                 public void createLocalCollection(Account account, ServerInfo.ResourceInfo resource) throws ContactsStorageException {
                     @Cleanup("release") ContentProviderClient provider = getActivity().getContentResolver().acquireContentProviderClient(ContactsContract.AUTHORITY);
                     LocalAddressBook addressBook = new LocalAddressBook(account, provider);
+
+                    // set URL
+                    addressBook.setURL(resource.getURL());
+
+                    // set Settings
                     ContentValues settings = new ContentValues(2);
                     settings.put(ContactsContract.Settings.SHOULD_SYNC, 1);
                     settings.put(ContactsContract.Settings.UNGROUPED_VISIBLE, 1);
diff --git a/app/src/main/res/values-de/strings.xml b/app/src/main/res/values-de/strings.xml
index 3aa4c9ca..9a20476a 100644
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -143,7 +143,12 @@
     <string name="settings_carddav_vcard4_support">VCard 4.0-Unterstützung</string>
     <string name="settings_carddav_vcard4_supported">Kontakte werden als VCard 4.0 gesendet</string>
     <string name="settings_carddav_vcard4_not_supported">Kontakte werden als VCard 3.0 gesendet</string>
-    
+
+    <string name="settings_android_update_title">Neue Android-Version</string>
+    <string name="settings_android_update_description">Die Änderung der Android-Version kann einen Einfluss auf DAVdroid haben. Falls etwas nicht funktioniert, löschen Sie bitte Ihre DAVdroid-Accounts und fügen Sie sie neu hinzu.</string>
+    <string name="settings_version_update_title">Einstellungen aktualisiert</string>
+    <string name="settings_version_update_description">Interne Einstellungen wurden aktualisiert. Falls etwas nicht funktioniert, löschen Sie bitte Ihre DAVdroid-Accounts und fügen Sie sie neu hinzu.</string>
+
     <string name="setup_select_collections">DAVdroid: Ordner auswählen</string>
     <string name="setup_neither_caldav_nor_carddav">An dieser Adresse konnte kein CalDAV- oder CardDAV-Dienst gefunden werden.</string>
     <string name="setup_add_account">Konto hinzufügen</string>
@@ -178,9 +183,9 @@
     <string name="setup_read_only">schreibgeschützt</string>
 
     <string name="sync_error_title">Synchronisierung von %s fehlgeschlagen</string>
-    <string name="sync_error">Fehler beim %1$s</string>
-    <string name="sync_error_http_dav">Serverfehler</string>
-    <string name="sync_error_local_storage">Datenbank-Fehler</string>
+    <string name="sync_error">Fehler beim %s</string>
+    <string name="sync_error_http_dav">Serverfehler beim %s</string>
+    <string name="sync_error_local_storage">Datenbank-Fehler beim %s</string>
     <string-array name="sync_error_phases">
         <item>Vorbereiten der Synchronisierung</item>
         <item>Abfragen der Server-Fähigkeiten</item>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 8afc501f..5e90267f 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -159,6 +159,11 @@
     <string name="settings_carddav_vcard4_supported">Contacts are sent in VCard 4.0 format</string>
     <string name="settings_carddav_vcard4_not_supported">Contacts are sent in VCard 3.0 format</string>
 
+    <string name="settings_android_update_title">Android version update</string>
+    <string name="settings_android_update_description">Android version updates may have an impact on how DAVdroid works. If there are problems, please delete your DAVdroid accounts and add them again.</string>
+    <string name="settings_version_update_title">Settings have been updated</string>
+    <string name="settings_version_update_description">Internal settings have been updated. If there are problems, please delete your DAVdroid accounts and add them again.</string>
+
     <string name="setup_select_collections">DAVdroid: Select collections</string>
     <string name="setup_neither_caldav_nor_carddav">No CalDAV-/CardDAV service is available at this location.</string>
     <string name="setup_add_account">Add account</string>
@@ -193,9 +198,9 @@
     <!-- sync errors and DebugInfoActivity -->
     <string name="debug_info_title">Debug info</string>
     <string name="sync_error_title">Synchronization of %s failed</string>
-    <string name="sync_error">Error while %1$s</string>
-    <string name="sync_error_http_dav">Server error</string>
-    <string name="sync_error_local_storage">Database error</string>
+    <string name="sync_error">Error while %s</string>
+    <string name="sync_error_http_dav">Server error while %s</string>
+    <string name="sync_error_local_storage">Database error while %s</string>
     <string-array name="sync_error_phases">
         <item>preparing synchronization</item>
         <item>querying capabilities</item>
diff --git a/app/src/main/res/xml/settings_account_prefs.xml b/app/src/main/res/xml/settings_account_prefs.xml
index 7198fb12..9fc0eeed 100644
--- a/app/src/main/res/xml/settings_account_prefs.xml
+++ b/app/src/main/res/xml/settings_account_prefs.xml
@@ -59,15 +59,10 @@
 
     </PreferenceCategory>
 
+    <!--
     <PreferenceCategory android:title="@string/settings_carddav">
 
-        <CheckBoxPreference
-            android:key="vcard4_support"
-            android:title="@string/settings_carddav_vcard4_support"
-            android:persistent="false"
-            android:summaryOn="@string/settings_carddav_vcard4_supported"
-            android:summaryOff="@string/settings_carddav_vcard4_not_supported" />
-
     </PreferenceCategory>
+    -->
 
 </PreferenceScreen>
diff --git a/dav4android b/dav4android
index 6fc7f30b..429648cf 160000
--- a/dav4android
+++ b/dav4android
@@ -1 +1 @@
-Subproject commit 6fc7f30b614ecb23b6fa66b6fd848fd6ddc4bafe
+Subproject commit 429648cfae534dd0476f813c94ab1cae1f140223
diff --git a/ical4android b/ical4android
index 62428770..662c48c4 160000
--- a/ical4android
+++ b/ical4android
@@ -1 +1 @@
-Subproject commit 624287708eb7f093d2ddf2012e2c2c124cd6e206
+Subproject commit 662c48c40ad66e9a77f4f4792587dc04ecc4a74e
diff --git a/vcard4android b/vcard4android
index 83ba3128..18b26fe4 160000
--- a/vcard4android
+++ b/vcard4android
@@ -1 +1 @@
-Subproject commit 83ba3128913ba6e57d021ad003bf992100baae43
+Subproject commit 18b26fe48896b2683b1e04e8df4464f5ceacf8f3