Bug fix related to .htaccess
The default .htaccess shipped with CMSMS uses:
RedirectMatch 403 ^.*/modules/(.*)\.php$
But the Responsive File Manager needs to be called by TinyMCE. Here's the new rule:
RedirectMatch 403 ^.*/modules/(?!TinyMCE/responsive_filemanager/filemanager/).*\.php$
So you'll have to edit your htaccess file if you want to use TinyMCE with the filemanager.
Updated help to add that config change
Thanks to Przemysław Kudłacik (pkudlacik)