I've seen 500 errors quite often when I have an .htaccess file that isn't configured properly. I believe that the .htaccess file often has a section that indicates what ErrorDocument to use when an error is encountered, so i think it is suspect.
I would suggest making a really simple php page that only has phpinfo() in it. See if you can load that with the current configuration. If not, try renaming the .htaccess file and see if it loads. If so, that file needs to be checked and customized for your system.