If you are using NTLM provider only for Windows Integrated Authentication for your web app, and the web app web.config has 401 custom handler, you may run into 401 and infinite loop of user credentials prompt.
To address the issue, check the web.config and see if you have the following and comment it out.
<!--error statusCode="401" path="/error.html" responseMode="ExecuteURL" /-->
The problem is when 401 is detected, it is routed to the custom handling page but the custom error handling page will get 401 too so you may run into infinite loop of user credentials prompt and 401.
