Hi Gossler,
Thanks for sharing this screenshot, however, it will be more helpful if you can share a video screencast showing what happens when you click on “X > Cornerstone” as I mentioned in my previous reply.
From this screenshot I can see that the main post editor is missing! this is most probably due to a plugin conflict, so please deactivate other installed plugins than Cornerstone and recheck this issue, if everything worked fine, then you can reactivate the plugins one by one till you figure out the culprit.
I would start by deactivating “Visual Composer” plugin because the theme is compatible with this plugin:
“WPBackery page builder” which was formerly known as Visual Composer, but I believe you have this one installed not WPBackery page builder. If you want to install “WPBackery Page Builder” you can do that from (X > Validation > Extensions).
That’s how your “Add New” page should look like even if you have activated WPBackery Page Builder plugin:
Thanks.