This error can occur if you are running ASP.NET Web pages using the Visual Web Developer Web server, because the URL includes a randomly selected port number. Proxy servers do not recognize the URL and return this error.
To get around the problem, change your settings in Internet Explorer to bypass the proxy server for local addresses, so that the request is not sent to the proxy.
In Internet Explorer, you can make this change in Tools > Internet Options. In the Connections tab, click LAN Settings and then select Bypass proxy server for local addresses.