This issue should be fixed with the version 4.0 upwards 

Try Possible Fixes

  1. Login to ‘File Manager’, and navigate to wp_config.php under your website main folder. Edit the file and add
    define('CONCATENATE_SCRIPTS', false);  after line define('db_collate'); .Save edited file and return back to the admin section of your website. Clear the cache using a plugin, or simply using the browser’s clear cache function. Try to open again the “Advanced Infobox”. This is one of the most common fixes of the rich html editor. (***This fix was tested, and worked for some users***)
  2. Create a new user with administrator rules. Log out and log back in with the new created admin user. (***This fix was tested and worked for some users***)
  3. Open TinyMCE Advanced settings page and click save. (It will reset the options) (only if you have TinyMCE Advanced installed)
  4. TinyMCE folder “wp-includes\js\tinymce.js” has to have 755 folder permission
  5. Go to Users->User Profile and uncheck “Disable the visual editor when writing” (***This fix was tested and worked for some users***)