Hi Jelle,
Thanks for your update.
Based on my experience, it might be caused by the SMTP matching. Could you please check if there is an on-premise user whose primary email address is beheerder@federateddomain.com? If there is such a user, please try logging into Office 365 by using the UPN of this user.
If this is not the scenario, I’d like to collect some tenant information for the issue. To protect your privacy, I have sent you a private message. Here are the steps to access the private message:
1. Please go to Your details section on the right side of the community site.
2. Click Private messages.
3. Click the subject title to read the message and reply with your information.
Best Regards,
Bruce Zhou