I don't have a clue
But wanted you to know that somebody noticed your post, Johannes. About all I can come up with are the generic things everybody knows, like rebooting.
I never upgraded to Windows 8, and I'm holding off on 10 while I watch the experiences, good and otherwise, that other people have. How an upgrade treats your existing accounts is one of the questions that always worries me, so I sympathize. Good luck.