We had the issue where Outlook 2016 kept prompting for a password for an Office 365 user whose laptop was connected to their Office 365 account Outlook OST vs PST repair using Windows 10 Pro. The repeated issue logging in started after I enabled the modern authentication in the Office 365 administration panel. The user could use the Windows mail and calendar apps but Outlook wouldn’t connect and kept prompting for a password. Checking the Account in Word showed the correct authentication and subscription. In the instances that this occurs the effected user is not new to MFA but has been running it for at least months.

Microsoft 365 Login Loop because of MFA

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. I experience the issue myself as well, every once in a while.

• I have a valid MS 365 subscription that includes 5 device licenses.
• Right now we have it set to prompt when a user is not on any of our corporate networks or they are using a mobile device.
• The user had Office 2015 and Office 2016 credentials and Teams credentials that we deleted.
• Tried CC cleaner to clean up registry and everything -- also keep in mind this is a fairly recent reinstall of windows.
• I created a new Outlook profile, and then the old one started working.
• On the main machine where I have been using the suite, now my account is deactivated and if I try to authenticate, it looks like it works but then get the "Fix", "Authenticate", "Activate" or "Resolve" buttons, all of which do nothing.
• I’ve seen this happen when the time and or date is wrong on the machine.

Fix constant sign-in prompt for Microsoft

In each case the fix has either been one of the how to fix Outlook not opening today above items or a combination of them. There doesn’t appear to be any pattern to the issue occurring or the problem being limited to any specific computer or user. So figuring that was the same issue here, I did the same thing.

Outlook stuck in an MFA loop

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.

Office 365 desktop apps infinite authentication loop -- asking to reintroduce details forever / activate office

We are looking at using conditional access policies where a user with a Domain joined PC is not prompted for MFA. Right now we have it set to prompt when a user is not on any of our corporate networks or they are using a mobile device. The user had Office 2015 and Office 2016 credentials and Teams credentials that we deleted. We checked with Word that the account was connected–it Fix Outlook Not Opening After Windows Update was and the we opened Outlook and it immediately connected. We too have been seeing this more and more, all W10 on O365, although we use Okta as our MFA.