Hi Clemens,
Can you confirm via the portal if you have roles and permissions assigned to the account? Might be some policies might have been applied during the testing and revoked your access check the permissions via this page - https://learn.microsoft.com/en-us/azure/role-based-access-control/role-assignments-portal?tabs=delegate-condition
Also check if any conditional access policy is invoked and logs from the AAD Auth logs might give you some pointers.
Hope this helps.
JS
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