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

The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action. Create your account and connect with a world of communities. I cannot find the phone number again to contact MS Office support (eventually they were helpful, but only after insisting they help!), I'm in the Netherlands so if someone has a phone number, I'd love to get it again.

Microsoft Office 365 Endless Authentication Loop Issue

• We’ve got a support case opened with Microsoft on this one but it’s been open for about 3 months.
• There doesn’t appear to be any pattern to the issue occurring or the problem being limited to any specific computer or user.
• It happened right before lunch, locked my machine, went out for lunch, came back, and logged in, this time successfully.
• I checked my MS account info page, subscription is fine.
• The user could use the Windows mail and calendar apps but Outlook wouldn’t connect and kept prompting for a password.

You may have to rebuild/restore your outlook folder/files which is a pain. I’d do that one step at a troubleshoot Outlook synchronizing folders error 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

I do NOT have this issue with the other laptop, an iPad, a MacBook Pro and FixTechGuide Office 365 troubleshooting tablet. So figuring that was the same issue here, I did the same thing. I'm also getting Error 29, which the MS support rep couldn't explain to me. I even tried disabling my anti -virus while authenticating, that didn't work.

Outlook 365 Password Prompt Issue on Windows Desktop

You can reply to reactivate the question if you decide you want to continue. Let's flush DNS and re-sync Outlook start by answering a few simple questions to understand the problem. We have been experiencing the exact same issue for months now. I experience the issue myself as well, every once in a while. On the off chance that none of these things solve this issue for you, please contact our support team at