Proxy servers are no more then cache's on your ISPs server....often visited sites have their content in that cache...so when a client request that site...instead to retriving it from the actual site...they are served from the cache...giving an illusion of faster browsing...
..after the page is served to the client...cache get enough time to actually connect to the site...and server any further requests...
Proxy server settings are the additional/optional settings.... which ISPs provide for better browsing....
If you can dial in into your ISP...and open up a browser...try some other sites...and see if you get the same response..
then
1- try disableing the proxy server settings.....for a direct connection to the web site in question..
Talk to your ISP ...about this problem....maybe they have changed the proxy servers setting or you need some extra settings....like a different port no ...etc...etc....
hope it helps..