Just finished testing and this one is working!
You can consider this one solved.
However, if you need some more information on this, let me know.
Thank you for the support.
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?
FIX: The old version was 0.9.x
UPDATE: Meanwhile things became worse. I removed the accounts and finally also reinstalled DAVdroid, because the calenders disappeared. The account is there in the system settings and also has contacts, calenders and tasks which can be synchronized, but the native calender app can not see any calenders linked to this account anymore.
UPDATE: Ok, after hours reading log files and other posts I happily found the solution. I have CyanogenMod and after reinstall DAVdroid automatically the Privacy Guard is switched on. Turn it off in the app settings! Everything seems fine now again.
Please put this hint into the FAQ for CyanogenMod users