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

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?

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.

Some reason (I suppose from an older Microsoft article) the registry key HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity - EnableADAL was set to 0. This key hadn't been changed manually before Outlook started choking. Maybe Outlook 365 was enabling modern authentication by default (which makes sense), and ignoring the key. Why it started paying attention to the key, I don't know.

I set it to value = 1 and Outlook logged in immediately.

I've tried both values for EnableADAL and it still gives the continuous prompts (including removing it). We don't have that value by default. I believe that is for Exchange Online. We run Exchange on premise. We set the value of EnableExchangeOnPremModernAuth=1. We enabled Modern Authentication on the server, but we disabled it on the mailboxes by policy because we found that Outlook cannot support a profile with more than one account if you have it enabled. We spent a long time with Microsoft support to finally figure that out. We did all this about a year ago, so it is not the cause.

Have you checked the Application Log in Event Viewer for Outlook related information? I was able to find this as an informational event:

The Exchange web service request GetAppManifests failed. The error code is 0.

HTTP Response Code: 401

Additional Error Message:

An unknown internal error occurred. The error code is 80004005

Have you seen these registry entries to try:

Manually Reset Autodiscover & EWS Settings

• Close Outlook.
• Open Registry Editor (regedit).
• Navigate to:

    CopyEditHKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
  

• Add (or modify) these DWORD (32-bit) Values:
  • ExcludeHttpsRootDomain = 1
  • ExcludeScpLookup = 1
  • ExcludeSrvRecord = 1
  • ExcludeHttpRedirect = 1
  • ExcludeSrvLookup = 1
  • ExcludeLastKnownGoodUrl = 1
  • PreferLocalXML = 1
• Restart the PC and try signing in again.

https://learn.microsoft.com/en-us/previous-versions/troubleshoot/outlook/unexpected-autodiscover-behavior