A 403 error means you tried to access a page that you don't have access to retrieve through a web browser. Is there any chance you have a .htaccess file in that directory with a setting that is preventing you from accessing that page?
A lot of times you see a 403 error when you try to get a directory listing for a directory in which Indexes have been disabled. Maybe you have
in the .htaccess file in a directory for which you don't have an index.html (or index.php or index.htm) file? That seems unlikely, though, since you say you are trying to access mt.cgi, and not a directory listing.
I think that the 404 error you saw just means that you don't have a custom "Page Not Found" ErrorDocument page, so the Apache web server just sends back a generic error page. You can specify a custom error page in a .htaccess file.