With advanced security configured web clients get pop up asking for username and password. Most username/password combos fail and do not provide access. Advanced security has been checked and is configured correctly.
The Application pool identity has insufficient rights
After verifying the IIS security settings are as they should be, check the Identity of the user running the App Pool. Try changing it.
6.0 Service Pack 1; 6.0 Service Pack 2; 6.0; 6.1; 6.1 Service Pack 1; 6.1 Service Pack 2; 6.2; 6.2 Service Pack 1; 6.2 Service Pack 2; 6.3