Which is what I suspected. (well, I actually expected a different URL, but I suspected the 500 error, or possibly a 403 (access forbidden, if it was trying to execute a file that didn't have execute permissions) )

To confirm that bassplayer's and my suspicions are correct, you'll want to continue with the rest of my suggestion:

If that wasn't useful, check your web server's error logs.

You really should check your webserver's error log, as there's most likely something more informative in there. (well, maybe not, as it might throw some misleading errors if it's trying to execute something that's not an executable file.)