@info001 you are brilliant. This has worked perfectly. Please tell us how you found it.
As the Doctor would say, "I came the long way around".
I was not willing to follow OldAJAS advice and dump a working system.
After 4 months it was clear that MS does not fix the Problem (I also tried the latest Win11 Insider-Build).
Thank God the problem also exists in my VMware clone.
As all my Win10 Backups had the same Problem, I used an old Win7 pre-Win10-upgrade backup.
Upgraded it with the same DVD as my PC.
Applied all available Patches form my PCs Update History (some unimportant were not available in Microsoft Update Catalog).
After that I was able to exchange parts form the two installations.
As the two installations share the same base (SAM/Security) they are partially compatible.
After spending huge time with CBS (Component-Based Servicing) I was able to exclude the filesystem as the problem.
It had to be the registry.
I suspected the SOFTWARE part or the COMPONENTS part of the registry.
By exchanging registry files between the two installations, I was able to exclude both.
I started to test all registry files in %SYSTEMROOT%\System32\config.
Bingo - "SECURITY" caused the Problem.
Because both installations share the same base, there were only few differences between them.
The rest was trial and error.
I've spent a lot more time on this than I care to admit.
Did anyone have problems afterwards?
Maybe with logins to MS-Cloud Accounts (I don't use them).
The deleted Keys are used to decrypt cached Domain-Passwords (according to doctor google).
Does anyone have tried the solution for a domain-joined Win10 ?
I can't thank you enough for this fix that MS has not addressed ever since they screwed up all subsequent updates in August 2022. I had posted this issue on this forum as well as Elevenforum and no one was able to figure out what the root cause was. Finally, someone on Elevenforum alerted me about your fix and I applied it to my machine and it worked. Hoping MS will take the ball and run with it and come up with an official solution. Thank you so much, you're a lifesaver and you don't realize how much it means to thousands of people who have been dealing with this issue for several months now. Keep up the great work!