Tim over at hotsinglegtrack.com passed this little snippet of information my way. If you’ve already tried disabling and enabling the RTE in a user’s profile without success, try this.
Try browsing to http://yourdomain.co.uk/wp-includes/js/tinymce/tiny_mce_config.php. If you get a 403 or 404 error your host might be prohibiting browser access to the file. You need browser access to it. If you can reach http://yourdomain.co.uk/wp-includes/js/tinymce/tiny_mce_gzip.php in your browser, try the following.
Locate the file tiny_mce_config.php in wp-includes/js/tinymce/and rename it to something like tiny_mce_configsafe.php. Edit /wp-includes/script-loader.php to reflect this change. Away you go.
This solution fixes the RTE if your host prevents the server from serving files that match the “*config*” pattern to the browser.
It worked for me, but YMMV.