I was able to solve the problem, in fact with the help of a Google AI bot. Without AI assistance, I wouldn't have found this error, even though I'm a computer scientist and have known Windows since version 3.0.
Apparently, you really have to ask the AI the right questions to arrive at a solution.
In my case, it had to do with user data that probably occurred during the clean installation last weekend…the installation likely got stuck somewhere unnoticed.
You have to delete a hidden folder, and to do that, you first have to run some command-line commands in PowerShell as an administrator.
However, I don't believe that yesterday's AI solution would also work with Windows 11, because according to the AI, Windows 11 uses services in some places that are structured differently than in Windows 10.
In addition, this solution requires command-line knowledge, such as PowerShell, among other things.
Therefore, I have some reservations about posting the solution here, because the problem is quite specific.