This is not the only fix for the MFA looping issue, but it does work. Use as last resort.
Other fixes before attempting are:
-clearing credential manager
-making sure work email authentication is functional in Windows settings
Using the below fix will cause Outlook user profile to be wiped and restarted (downloads all info again). Personal folders and such will need to be reconnected.
Back up registry keys before deleting!
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity