The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map
error is usually caused by a few different things:
invalid code/characters in the .htaccess file
invalid file permissions on the index.php file (ie - can't have permissions higher than 0644 on files when running suPHP)
too many processes running (ie - cpanel limits an account to 25 processes [php, imap, mysql, etc]