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

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

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.

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.

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.