theme-editor forbidden error

I have a very strange problem with the theme editor. I can edit some files fine – for example, style.css, page.php, index.php. However, when I try to edit header.php I get the following error:

Forbidden
You don’t have permission to access /s/wp/wp-admin/theme-editor.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I really can’t understand why it does this – everything’s CHMOD’d correctly (666). The theme is one I made myself but should that have any effect – I wouldn’t have thought so?

Thanks