I experience the issue myself as well, every once in a while. Tried CC cleaner to clean up registry and everything -- also keep in mind this troubleshoot Outlook synchronizing folders error is a fairly recent reinstall of windows. I am unable to stay signed in to my work account on Teams, Outlook, Office365 for more than 5 minutes.

Outlook credential prompt loop

In the instances that this occurs the effected user is not new to MFA but has been running it for at least months. The problem appears to randomly affect users (I’ve had it affect myself once too), and the only solution appears to one of (or a combination of) the options in the OP. Out of 140 staff the problem seems to occur once every 2-3 weeks on average. 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 FixTechGuide fix for stuck folders the profile and clearing the credential manager.

Loop of 'Sign in to your account'.

I had this exact issue on another machine, that after 2.5 hours on the phone with MS support (via Zoom), the solution was to uninstall MS Office 365 and reinstall it. 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’. I created a new Outlook profile, and then the old one started working. I wonder if the compuer cached a new logon token or something that the old Outlook profile could use. Also, during the new profile creation, I was only prompted for a password and not for MFA.

Resetting the Windows 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. We’ve got a support case opened with Microsoft on this one but it’s been credential loop in Microsoft 365 open for about 3 months. Unfortunately when the problem occurs they’ve asked us to ring straight away but of course you can never get them instantly (we’ve just been given someone’s direct number now). To make a long story short we’ve since been asked to collect Outlook logs and install Fiddler to capture additional logging. Initially, this account didn't have MFA turned on, and I was able to log in.

• It’s not caused by an out of sync MFA prompt as I have personally looked at a number of cases with the user as stepped through it.
• 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.
• 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.
• Not sure if it has to do with being an onmicrosoft.com account that has no domain assigned to it.
• I have a valid MS 365 subscription that includes 5 device licenses.
• Checking the Account in Word showed the correct authentication and subscription.
• Office 2016 is already Modern Authentication aware so APP passwords are not required.
• Native IOS and Android Mail apps will require the APP password or users can use Outlook for IOS/Android which is Modern Auth aware.

Changing Outlook Security Settings:

If this box is greyed out or disabled then you need to reset the Windows Credential Manager first. @Microsoft, given the level of attention the community has given this issue, I hope you come up with an update soon. We have been experiencing the exact same issue for months now.