Maybe the server is sending a wrong HTTP-header...
I you have Firefox installed on you system, use the "Live HTTP headers" extension in order to have a look at the delivered headers.
A normal header should look like that:
HTTP/1.x 200 OK
Date: Thu, 11 Dec 2008 20:13:16 GMT
Server: Apache/1.3.37 (Unix) PHP/4.4.8 with Suhosin-Patch FrontPage/5.0.2.4803 mod_fastcgi/mod_fastcgi-SNAP-0404142202 mod_ssl/2.8.28 OpenSSL/0.9.6i
X-Powered-By: PHP/4.4.8
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Keep-Alive: timeout=1, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html