After an update to Owncould 9.0.1 and use from DAVdroid 1.0.7-beta-1 every think working.
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