MFA policy is misconfigured - Unable to Login to admin panel

A customer had previously deactivated MFA for his user. now he wanted to activate it. He clicked on his user in the admin center, clicked on manage MFA and changed the option to force. When he logs in to office.com he gets the error message:

MFA policy is misconfigured - Unable to Login to admin panel

Does anyone have a tip for me as to what this could be?