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Dear Georg,
Thanks for writing back and updating us. Apologies for the delay in the response.
As you mentioned the problem happens with other colleague, I suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t the admin of your Office 365 organization, please contact the admin of creating a new ticket.
Appreciate your patience and understanding with us. Have a good day!!
Best Regards,
Mia