torvista, do you happen to have the SuperGlobals plugin installed? I'm wondering what the information in your store's $_ENV and $_SERVER are for the request-related information.

I'm successfully using the previous version of this plugin on a 1.3.9h store with Ceon URI mapping, so that's not it.

Does it make a difference if you change
Code:
    $email_text = sprintf(EMAIL_PAGE_NOT_FOUND_CONTENT, $_SESSION['customers_host_address'], $_SESSION['customers_ip_address'], $_ENV['REQUEST_URI'])."\n\n";
to
Code:
    $email_text = sprintf(EMAIL_PAGE_NOT_FOUND_CONTENT, $_SESSION['customers_host_address'], $_SESSION['customers_ip_address'], $_SERVER['REQUEST_URI'])."\n\n";