Upgrading to Horde Framework 5.2.14 solved the problem.
Thanks for the tip.
Calendar VALARM is not synced
VALARM is not synced from/to owncloud.
While owncloud currently does not support VALARM, this field is still stored in owncloud’s ics when I sync a calendar entry from thunderbird/sogo.
Syncing with davdroid there is no such field. It would be great if this would sync all the way through phone->owncloud->thunderbird and vice versa.
sorry, I did not realize this is a duplicate of Issue #70