Outlook email keeps prompting for a password, and then not accept it. Blocked out of emails.

I've tried recreating the profiles as well many times with no success. We know for a fact that other applications (not Office) are experiencing authentication issues after we upgraded one of the domain controllers in our network to Windows Server 2025. We see others are having authentication issues as well after upgrading to 2025 DC's. We started to observe this Outlook issue about the same time and are wondering if they are related or not.

This problem started for me on 2-28-2025. I am using the latest updated version of Office 365 and Outlook opens but immediately prompts with a Windows Security window, Microsoft Outlook , Connecting to .... , User name text field, Password text field, Remember my credentials check box, OK and Cancel buttons.

Password entered correctly and click OK, and I can do this as many times as I want, over and over, and never get anywhere.

I have uninstalled Office completely and used Revo uninstaller to search for leftovers.

I deleted the Outlook profile after uninstallation.

I deleted every single saved credential in Credential Manager.

I removed the Microsoft Account in Settings on Windows 11.

I removed the saved Profile in Microsoft Edge and fully cleared the cache.

Reinstalled from the link inside my Office 365 account through Microsoft Edge. Created a new Outlook profile and was able to use Outlook for 1 day. Upon launch the next day, the same prompt appears.

This reminds me of something Microsoft SARA was able to fix, but that is discontinued now and the "Get Help" app only leads to web searches. When I was able to try starting a troubleshooting guide, it errored out with "Protocol not enabled". No clue what that means.

Subscription is Microsoft 365 Business Premium.

Outlook profile is linked to the Exchange account in the Business Premium account. That is the only email account in the Outlook profile.

Outlook is bricked. I really don't want to work with the web version. That just isn't a possibility. How can this be fixed?

Experiencing it too. Are these systems part of a Windows AD domain with a Windows Server 2025 DC?

Sorry I forgot to add the 1st paragraph:

I have been experiencing an issue with Classic Outlook for about a month now. Every time I open the application, I am prompted to enter my password repeatedly. The issue persists even when I try creating a new profile.

I can bypass the password prompt by clicking "Cancel," which allows me to access Outlook, but this is not a sustainable solution. Since then, I’ve been using the new Outlook or Outlook on the web without any problems, but they don’t fully meet my needs like Classic Outlook does.

Here are the answers to your queries:

Mailbox extension: .pst

Type of Account: Company/school use (Microsoft 365 account)

Software Version: Microsoft 365, fully updated (Version 2501 Build 16.0.18429.20044). I also ran an Office Online Repair today.

Protocol: IMAP

Additionally, I have already tried the following steps without success:

Cleared cached credentials in the Windows Credential Manager.

Verified Modern Authentication is enabled and configured properly.

Reset network settings.

Used the Support and Recovery Assistant (SaRA) tool for troubleshooting.

Ran Outlook in Safe Mode and attempted to disable add-ins.

Repaired Outlook data files with SCANPST.

The issue persists even with these efforts. Any suggestions or further steps I can try would be greatly appreciated.

PS.: I tried to inset a screenshot without success