IIS Authentication Pop up Issue

Problem : For no good reason, suddenly a perfectly fine working IIS on Windows decides to give the following browser authentication pop up which never validates any credential and eventually gives a 401 - Unauthorized access error.

An exactly identical server with same settings in the network could be working fine.

Solution:

Open Internet Information Services (IIS) Manager

Double click server name

Double click authentication icon

Select “Windows Authentication” and click “Providers” in the column on the right.

Select “NTLM” and Move it one up; above Negotiate, Click OK.

Notes : This has worked more than once for me. There is no known reason why the server decides to behave this way, but it may have to do with the authentication handshake protocols that the server does at the back end. If your server is currently working fine, there is no need to change the setting.