Thanks for your tips and the links.
Finally, I give up. Extra account: error message. Clean boot: error message.
With all this testing and "cleaning" I found out that I had installed some weeks ago the 30 days trail, which got uninstalled while Win 10 Upgrade 1803 performed some further updates.
Maybe there's an easy solution if someone already hat Fences as trail installed?
If not, I'll give up and retry with the next Microsoft Update. I spent so much time on several possible solutions and not a single one worked - maybe the error within my OS will be "cleaned" with MS next upgrade.
I can wait!