@jmbreuer finally managed to get logs from DAVx5 - when I tried before, the “Logs” box underneath “Debug info” was not showing up.
This is a log starting with a completely empty DAVx5, adding my account (via URL), checking only the CardDAV address book and tapping the sync button on the bottom of the screen:
The thing that stands out to me is the “[syncadapter.SyncAdapterService] Sync thread cancelled! Interrupting sync” when I’m just letting the thing run it’s first round - race condition? It looks a bit like after the interrupted sync, DAVx5 assumes that first interrupted sync worked and only takes diffs from then.?