In this case are you saying that there is a password prompt for the users credentials? Clear the Windows Credential manager or recreate the users Outlook profile. In fact if you just turned on MFA and you were not using MFA before you usually end up having to rebuild the profile and clearing the credential manager. If you are saying that they are getting multiple MFA prompts then this is an out of sync issue with the wrong code being used.

Additional resources

Please help can't afford a new computer if I throw this one against the wall. Yeap, you describe it exactly and unfortunately we still don’t have an answer. Closed the window without entering anything and Outlook returned to ‘Connected to Exchange’. Office 2016 is already Modern Authentication aware so APP passwords are not required.

Microsoft Office 365 Endless Authentication Loop Issue

• Checking the Account in Word showed the correct authentication and subscription.
• FYI outlook 2013 requires a registry setting to work with MA.
• These versions have the Logon network security setting disabled or removed from the Microsoft Exchange email account settings.
• I have a valid MS 365 subscription that includes 5 device licenses.
• Modifying the Outlook profile by using the "Profiles" registry path is not supported and may cause your Outlook profile to be in an unsupported state.
• Have looked all over and people have all sorts of possible solutions, but nothing we found to resolve the issue and get a new issue daily and just keep going through until it eventually works.
• The user had Office 2015 and Office 2016 credentials and Teams credentials that we deleted.

You may have to rebuild/restore your outlook folder/files which is a pain. I’d do that one step at a mailbox not updating on IMAP time, forgoing the restore of the cached files. If this box is greyed out or disabled then you need to reset the Windows Credential Manager first. Or, any suggestion on how to fix this issue, even better.

Outlook stuck in an MFA loop

Close and open your web browser again and go into connection status shows disconnected SuiteFiles. You will most likely need to enter your Microsoft 365 credentials again so ensure you tick any box that says remember my credentials but it should now work for all future documents you open and not require your credentials. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. I regret that you are experiencing issues with your Microsoft installation.

Outlook 365 Password Prompt Issue on Windows Desktop

So not doing anything at all even seems to work. Which to me sounds online Outlook email repair utility like the issue is service side, not client side. Problem doesn’t just happen on PC’s connected to the internal network, but can happen when a laptop is remote and connected via external wifi or mobile hotspot.