That did it, thank you. I should have thought of that earlier.
I noticed that before I deleted and re-added the accounts, there was no way in the contacts application to add details for them in the few contacts that I created manually. You know, I could add a person and specify phone number and email address etc., but no XMPP-account. Now, after re-adding the accounts, I can.

So what happened before probably was something like: The N9 got the information from my online accounts, tried to store them in the contacts database but couldn't, because there were no corresponding data fields. That would explain the error I got.

Yes, they are still there, I checked. But given the experience with my online accounts (syncing is one thing, storing them in the contacts db another) it might well be that they're not synced at all. The sync framework could have noticed that the contacts on the server never changed since the last sync - and simply not pass them on to the contacts db.

I'll do what helped me in the past with sync issues: wipe the server and upload all the data again from my pc. They should look like brand new contacts to my N9. Let's see what happens then. (Only trouble is that syncevolution is in a sorry state on my desktop PC right now... need to give it some love...)