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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
I wonder if the compuer cached a new logon token or something that the old Outlook profile could use.I experience the issue myself as well, every once in a while.In each case the fix has either been one of the above items or a combination of them.I then approve the login with my authenticator app.Outlook must be online or connected to complete this action.Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support.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.
You may have to rebuild/restore your outlook folder/files which is a pain. I’d do that one step at a inbox repair tool alternative 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 Outlook profile repair and recovery steps 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
Microsoft Outlook 2016 and some recent builds of Outlook 2013 are not affected by this issue. Those versions have been updated to prevent the problem that is described in the "Symptoms" section. These versions have the Logon network security setting disabled or removed from the Microsoft Exchange email account Outlook profile repair and recovery steps settings. It requests me to login to my microsoft account. I then approve the login with my authenticator app. Then i get rerouted to the login page again to repeat the process.