I take it you've tried reverting to the Classic template and everything works then? Or not?

It isn't just your Add to Cart button that leads to a blank page - the other two buttons (Tell a Friend, Review) do the same.

Blank pages are usually caused by corrupted or improperly edited PHP files. Do any of the templates you tried modify any of the pages that are showing up blank? Maybe you have some corrupted files in template_default.