Problem solved!
First of all, thank you very much for the tip about the billing category. That finally allowed me to start a live chat with an agent.
After a brief exchange he accessed my PC remotely and was able to narrow down the error to the fact that windows could not establish a connection with the "organization".
I did not know exactly what that meant.
When i looked under "activcation" before, this error was not displayed to me either.
But the agent could activate my Win11 again and everything seems working fine again.