For me, turning off WPML fixed it temporarily. So I could use the editor as usual.
With WPML activated it threw this:
Failed to load resource: the server responded with a status of 404 (Not Found) https://xxx.xxx.xxx/wp-includes/js/tinymce/tiny_mce.js?ver=3.9
Uncaught ReferenceError: tinyMCE is not defined
