I have recently switched from ownCloud to Horde as my CardDAV server. Now there is a weird bug, which is also present when using the app "CardDAV Sync". I have already notified the developers there of the bug, and I figured I would do so here as well.
The problem is visible in the screenshots. You see an empty "pager" line for each of my contacts. There is even more "empty" fields that are synced from Horde: e.g. the private address, an empty Fax address, and a birthday of "1st of January 1970", i.e. the start of Unix time. This problem even happens on completely new address books that were just created in Horde.
The problem does NOT appear at first when a contact is created on Android in the DAVdroid account, but when the DAVdroid account is deleted and re-created, the wrong, "empty" information is again added to the contacts. So I suspect that Horde is sending data that should be interpreted as "there is no entry here", while DAVdroid understands it as "there is empty entry here". The bug does NOT happen with Desktop apps, so I suppose there might be different interpretations as to how to implement the CardDAV standard here - or perhaps it is also just a Horde bug.
On my server, I am running Horde Groupware 5.1.3, in case that matters.