Seems that you can only access this site from a pre-determinded IP address range.

HTTP Error 401
401.3 Unauthorized: Unauthorized due to ACL on resource

This error indicates that the credentials passed by the client do not have access to the particular resource on the server. This resource could be either the page or file listed in the address line of the client, or it could be another file on the server that is needed to process the file listed on the address line of the client.

Please make a note of the entire address you were trying to access and then contact the Web server's administrator to verify that you have permission to access the requested resource.