What does this site's "Bad Request" error mean?

For the last week or so, everytime I go to forums.hipinion.com, I get this error:

Bad Request

Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.
Cookie: __utma=24852781.665981920.1268694499.1282238477.1282445291.527; __utmb=24852781.1.10.1282445291; __utmc=24852781; __utmz=24852781.1268694499.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); phpbb3_5ouhf_k=; phpbb3_5ouhf_sid=009f2062971812b8e96912a5070ee005; phpbb3_5ouhf_track=tf%3A%281%3A%285vo%3A1...
Apache/2.2.8 (Ubuntu) mod_jk/1.2.25 mod_python/3.3.1 Python/2.5.2 PHP/5.2.4-2ubuntu5.9 with Suhosin-Patch mod_ssl/2.2.8 OpenSSL/0.9.8g mod_perl/2.0.3 Perl/v5.8.8 Server at default Port 80

I have no idea what it means, but it won't let me move past it. The site usually gets fixed within a day or two if it's down.

The error is explained in detail here, basically something on your side of the internet is screwing the URL request that the browser sends.
HTTP Error 400 Bad request Explained

How else can I fix it? I read that link and my cache's been cleared and it only happens on that one site.

You need to clear your cookies as this is the problem. Depending on the browser you're using, you may be able to remove the specific cookie. What browser are you using?

I'm using Safari. Cleared the cookies and the site worked for a few minutes then I got that error again. Which cookie should I delete and where is it?

No idea. Open up Safari preferences > Security > Show Cookies and search for hipinion. Delete all the hipinion cookies as a start.