Topic: Problems Editing header.tpl file

When I edit the header.tpl file, after saving I get the following error:

You don't have permission to access /directory/admin/adm_html_edit.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/1.3.37 Server at Port 80

I seem to be able to edit other files, but not this one. What do I need to do to correct this?