Thank you that link helped. The Problem is gone now.
However OpenTask should ask for permission instead of letting the user unknowing he has to request them by hand through the settings.
Phone unable to sync contacts due to SQLite error
After installing a new ROM into my phone and restoring the apps with Titanium Backup, I’ve been unable to sync contacts specifically; I was able to do so yesterday. The error is not appearing for the calendar or the tasks synchronization. The error seems to be caused due to an issue with the SQLite database being falsely marked as read-only. Neither reinstalling DAVDroid nor removing the data from the Contacts app solved the issue. Currently using Paranoid Android 7.1.2.
Is the file actually read-only (no write permissions)?