@Georg Heiler
Thank you for following up on this and I'm glad that you were able to resolve your issue!
Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer.
Issue:
After following the Single sign-on (SSO) quick start documentation, you ran into the below error message. The issue was that you tried to execute the script npm run configure-sso
and logged in with the demo admin account.
Error Message:
It looks like the application '<<id>>' you are trying to use has been removed or is configured to use an incorrect application identifier.
Solution:
You resolved this by adding a M365 tenant and logging in with your real account.
Thank you again for your time and patience throughout this issue.
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Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.