When I log in, I get a 32 hex-char sid (session id?) in the url.

It looks like this:
<http://giantesscity.com/forum/index.php?sid=0123456789abcdef0123456789abcdef>

Did you try putting that in the starting URL?Make sure the sid you use in the
url matches the sid that is in the cookie(s).

If you already have that, I'm afraid I can't be much help.  But if you didn't,
I would think that would be a good place to start...