There are few things that can cause 500 Internal Server errors. The most common causes are listed in the link below.
http://forum.ait-pro.com/forums/topic/read-me-first-free/
A couple other rare/not common issues that are pending being added to the Forum link above are below. Only consider these as possibilities after checking the things in the link above.
mod_security SecRule/SecFilter issue/problem.
Adding invalid .htaccess file to BPS Custom Code.
