Please see here: https://forums.bitfire.at/post/7538
Please make sure that your server doesn’t serve any VCard with the questionable REV time format (i.e. apply >the OwnCloud Contacts fix and then correct invalid VCards).
After that, synchronize all contacts again.
Step 2: To do so, unselect the address books in DAVdroid, force sync, wait a bit until the local contacts are empty and sync again.
@rfc2822 first off: thanks for the immediate reply. Second: wow! Birthday Adapter from F-droid is so cool! It is exactly what I was looking for. Thank you! And thank you for DAVDroid! (and sorry for the duplicate)