Can't say I've seen that error often. Does it come up immediately when you open the page, or when you are doing something on the forum?
Do you have access to your server error logs, and do they show anything that might be related?
I have seen that error happen with with other software sometimes, and if I remember correctly it had something to do with the formatting of the code. Any lines with <!--# in the beginning would throw that error, and removing the # would fix it -but what actually was to blame, I can't remember....