Safari not connecting to one site only

Ok, I'm stumped, so instead of answering a question I'll ask one!

Since yesterday afternoon I have been unable to connect to a particular forum website that I frequent. However, this is only on my iMac running Snow Leopard, my laptop with 10.5.6 connects fine.
This is the error I get:

Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit. Followed by a huge amount of text and numbers.

I have reset Safari, cleared the cache and restarted the iMac, all to no avail.
The only thing I have done on the iMac is a bit of browsing and I tried to install a printer driver (which failed, but that's another matter).

Any ideas? Thanks.

Scratch that then!
I just reset it again, and it seems to have sorted it.
It was an issue with a cookie, which I had cleared, but it seems it took a few resets!

And... it's happening again, so back to square one!

Any ideas, if it was a bad cookie it would happen on the laptop too, right?

No opinions/suggestions? Hmm...

?? - I've only seen that message once - and with Safari a couple of weeks ago. For me it only occurred at mac-forums. Had been online for awhile and started getting that same message. All other sites continued to work with Safari. FF & Opera still worked here. I did nothing and it just started working again after about 40 minutes or so.

Ok, I fixed this one, but I had to manually delete every cookie for the site concerned. It seems to be holding up ok, but I have to wonder why it did this. Obviously it's a Snow Leopard issue somewhere too.