Apache / CPanel - 500 Server errors
One possible reason for getting 500 "internal server errors" in a CPanel / Apache setup is the permissions on the public_html (and sub-folder structure).
The permissions should be 755 (all can read and execute, only user can write).
Owner: Read Write Execute (7 or 111 in binary)
Group: Read Execute (5 or 101 in binary)
Public: Read Execute (5 or 101 in binary)