“You do not have permission to view this directory or page using the credentials that you supplied because your Web browser is sending a WWW-Authenticate header field that the Web server is not configured to accept.”
This error will occur if you are trying to use NTLM authorization on a web site, and the users are hitting it from behind a proxy server. There is no 'fix' for this. The web site will need to be reconfigured to use basic or digest authentication.