OSX Server 10.4.10 – HTTP Error 401.2 – Unauthorized: Access is denied due to server configuration.

I am getting an intermittent error: HTTP Error 401.2 - Unauthorized: Access is denied due to server configuration. When I put in the URL, or even just the IP address, a user name/password box comes up It is the only site I am hosting on the server and if I start and stop the Web service , the page will display properly without asking for credentials. I am running Portfolio 8.5 Netpublish on the same server, but the error occurs even when the Netpublish server is stopped - Any ideas why this error is occurring?