I noticed the whole editor (buttons + content into the textarea) is not displayed anymore when the following cases are met:
- WP_CONTENT_URL is defined into wp-config.php
- Admin menu is browsed over HTTPS (FORCE_SSL_ADMIN set to true)
- WP_CONTENT_URL cannot serve files over HTTPS (only available via HTTP)
In fact I am using WP_CONTENT_URL as a CDN, so all my static files are served via the CDN over a non-encrypted connection.
Over HTTPS in the admin menu, all TinyMCE Advanced js and css components are set up to be also retrieved using the CDN URL on https, which is not enabled, so the editor is not displayed.
I'm not sure it is a bug, but since WordPress core seems to ignore WP_CONTENT_URL when the user is browing the admin menu (the admin menu style is correctly displayed), maybe plugins should do the same :)