When you say 'slightly customized' main_page, what exactly did you do, and if you replace that page with a fresh (stock) copy, does the site come back?

My guess is that you've introduced blank lines or spaces into define_main_page.php or used unescaped special characters somewhere.

You might try this, too:

http://www.zen-cart.com/index.php?ma...roducts_id=860