After yesterdays hard reset and todays bizzare attempt to get everything back to work I stumbled upon a strange Activesync 80070057 error. This one was not that hard as it initially looked - as all bizzare errors with strange numbers, I would like to see who is actually “inventing” these numbers =)
The cause for this is probably some incorrect binding between which folders sync and which don’t. Either create a new partnership or disable all synchronization tasks in Tools > Options. Restart. And reactivate them. Voila =)