Hi @David Bucha ,
Please let us know the below details,
1.What is the full version of Exchange Server?
2.Is it happening to all users or some?
3.Problematic user is able to login to OWA?
4.If the user is able to login to OWA, at what stage the prompt appears at outlook? Can you share the screenshot by covering your personal information
Check the authentication settings on the virtual directories
If the above suggestion helps, please click on "Accept Answer" and upvote it.