According to the logs:
EXCEPTION java.io.IOException: stream was reset: REFUSED_STREAM
this is a duplicate of nginx HTTP/2 incompatible with okhttp. Please follow up there.
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