Hi @Roman Dzichel, MP-Datentechnik GmbH
For our better research, please provide more information about this case:
- What does the error page look like and is there an error code?
- Can you log in properly in the browser client?
If your users encounter errors when they try to sign into Microsoft Teams, please try running the following tests to populate diagnostics in the Microsoft 365 admin center: Run Tests: Teams Sign-in.
If diagnostics don't detect any issues but you still can't sign in, we recommend that you try Microsoft Support and Recovery Assistant, which runs tests to find out what the problem is and provides the best solution to the identified issue.
Hope this helps!
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