There’s quite a few forum posts about this issue and the general consensus seems to be “Re-upload the wp-includes” directory.
While that seems to solve the problem for some, it didn’t for me, until I did some more poking around.
The actual problem in our case was that the editor in the admin center was trying load this file:
/wp-includes/js/tinymce/langs/en.js
That file isn’t contained the latest version of WordPress. The file that’s actually included with wordpress in that directory is called “wp-langs-en.js”
It wasn’t until I renamed that file to en.js that my editor finally started to work.
I think that the WP Development teams needs to take a look at why either the file is improperly named or why it’s being improperly called/linked by the admin applicztion.
