To use OpenTasks I reinstalled DAVdroid (deleted old <0.9, installed new 1.0.8) and recreated my accounts. I could see my calenders and lists but synchronization fails and a notification appears, if started from within DAVdroid. If I initiate the sync from my calender app nothing happens. The server side is Horde 5. The debug log:
SYNCHRONIZATION INFO
Synchronization phase: 9
Account name: ...
Authority: com.android.contacts
EXCEPTION:
at.bitfire.vcard4android.ContactsStorageException: Couldn't apply batch operation
at at.bitfire.vcard4android.BatchOperation.commit(BatchOperation.java:53)
at at.bitfire.vcard4android.AndroidContact.add(AndroidContact.java:605)
at at.bitfire.davdroid.syncadapter.ContactsSyncManager.processVCard(ContactsSyncManager.java:281)
at at.bitfire.davdroid.syncadapter.ContactsSyncManager.downloadRemote(ContactsSyncManager.java:226)
at at.bitfire.davdroid.syncadapter.SyncManager.performSync(SyncManager.java:170)
at at.bitfire.davdroid.syncadapter.ContactsSyncAdapterService$ContactsSyncAdapter.onPerformSync(ContactsSyncAdapterService.java:66)
at android.content.AbstractThreadedSyncAdapter$SyncThread.run(AbstractThreadedSyncAdapter.java)
Caused by: android.content.OperationApplicationException: insert failed
at android.database.DatabaseUtils.readExceptionWithOperationApplicationExceptionFromParcel(DatabaseUtils.java)
at android.content.ContentProviderProxy.applyBatch(ContentProviderNative.java)
at android.content.ContentProviderClient.applyBatch(ContentProviderClient.java)
at at.bitfire.vcard4android.BatchOperation.commit(BatchOperation.java:44)
... 6 more
SOFTWARE INFORMATION
DAVdroid version: 1.0.8 (99) Wed Apr 27 09:44:38 CEST 2016
Installed from: org.fdroid.fdroid
JB Workaround installed: no
Do you need further information?