Single sign on
401 - Unauthorized: Access is denied due to invalid credentials.this.
This error can occure after a server reboot, and if there has been a Windows update patch prior to the reboot.
The site is set up with Windows authentication, but we are getting the Windows logon box repeatedly until the error message above appears. The solution to this is to check the Provider used on the Winsows authentication in IIS.
Right click on the Winsows authentication and select Provider
Move the NTLM Provider to the top and save.