Does it happen when you click to start a new thread or when you try to post the new thread? Also do you have any addons/plugins for your browser? Namely things like Greasemonkey with scripts running? These are the primary issue from what I found.
It's strange because the error seems to be on the side of the website from what I saw when I looked it up. It looks like you have around 32MB (which is standard) to make the post which is A LOT of text. Something is eating it up somewhere...