Since several DAVdroid versions after an update to the app syncronization to OwnCloud 8.x (production version setting) gives errors. The other way (OwnCloud calendar to DAVdroid seems to work).
From other threads I read this could be fixed with an upgrade to oC 9, but it seems 9 is half a year or more away from becoming production.
The immediate workarround by DAVdroid seems to delete and resetup the calendar. But this seems not enough. Un- and reinstall of DAVdroid seems to help (until the next app update).
Not sure how to handle this, maybe by blocking any DAVdroid app updates.