Ok, seem I resolved my SSL certificate problems, despite the CA-Flag.
At sometime finding the bug i disabled the StartCom certificate in the System Settings.
Enabling it and going through accept permanently dialogs seems to fixed it.
Calender gets synced again. .-)
Disable error message
How can I disable the error messages. My home server is off during the day. Therefore, it would be an advantage that no error messages appear. Thank you.
You can either activate Wifi only sync and enter your specific home wifi or you can disable all notifications for DAVdroid in Android Settings -> Apps -> DAVdroid -> Turn off notifications