Well, there are no error logs from DAVdroid. It all happens after the sync. Switching on explicit logging does now show anything out of the ordinary.
Nothing in the FAQ or config guide would explain what I am seeing.
So I think it must be related to the contacts storage/account.
The steps I took where activating the sync of the SoGO address book for an already existing configuration that I use to sync the calendar.
This gives me an account to store contacts in on the phone.
I configured contacts (and also contacts+) to show the address book entries for the sync account.
Sync runs through without errors.
But, I never see the contacts in that account.
While I can create them just fine on the phone for this account (DAVDroid contact), they never show up on the phone (only after the initial entry, the whole contact is shown, but as soon as I go to the screen that lists contacts they vanish), but they do get synced to SoGO, so I see them in the SoGO Web interface and the Mac Contacts app that also syncs with SoGO. This proves that the sync process is working.
But none of address book entries for the synced account show up in any address book app I tried.
This was yesterday.
Today I tried it slightly different, I switched of periodic sync and set the sync to manual.
In that case, the newly entered contact on the phone is visible until I manually sync, then it vanishes from the phones contact list. It shows up fine on the SoGO Server.
I had the debug log switched on, but it only shows that one entry for the address book was synced.