Here is a page that says that "UBB.Classic" requires CGI and Perl 5.004. Of course it still looks like the page is PHP. The linked page also says:

OPTIONAL: PHP 4 or higher - In order to use the UBB.classic™ Accelerator option, which can help enhance performance, you must have PHP 4 or higher.

I have no idea what the code looks like or how it was modified so it's hard to help figure out what is going wrong. If you think it's related to code you changed then trace through it especially near any database updates, etc.

Finally if this is just a PHP question I think it should be deleted. I'm sure there are more appropriate places for that sort of question.