/PORT option. The HTTP protocol is a telnet protocol, so you can simulate it just by typing things in. This will help you to see exactly what a sending back, and it will check you that it really is the server not the browser which has a problem.
Here is a simple example (keybord input is in boldface):
> telnet myhost.domain 80 Connected to myhost.domain on port 80 Escape is ^[ GET /document/url ...document or error message...