I am still a happy user of DAVdroid with Nextcloud on my phone running LineageOS. I have a contact that fails to upload after a change on my phone, here is the DAVdroid log.
You can see that the server returns
415: Unsupported media type (REV MUST NOT appear more than once in a VCARD component). Indeed, REV field is present two times for whatever reason.
I do not know the CardDAV specification, but would it be possible for DAVdroid to detect such issue in the VCARD, delete one of the REV field (I suppose the better would be to delete the oldest), and finally upload the well-formed VCARD?