Hello @Mikael Puusaari , the Microsoft Entra ID (formerly Azure AD) 50074 error is being caused by the lack of strong authentication as required by your tenant MFA settings. This is why you were getting the More information Required prompt. Althought your findings suggest that switching to System Web Browser may have helped, my guess is that you could finally register and pass MFA or that MFA settings were relaxed. For more information please reach your tenant security admin or write us to azcommunity@microsoft.com with Subject Attn: Alfredo Revilla.
Let us know if you need additional assistance. If the answer was helpful, please accept it and rate it so that others facing a similar issue can easily find a solution.