Hi @Roland Makkelie , you're experiencing the AADSTS50020 error. This means that the user you're trying to sign is hasn't been properly added as an external user. For example, if the user is using a Google or Facebook domain you'd have to register them appropriately. You can invite an external user by following this document. Please let me know if you have any questions and I can help you further.
If this answer helped you please mark it as "Verified" so other users can reference it.