I am having the same problem - Edit with Cornerstone does not load.
I can see these error in the JS Console:
TypeError: window.csGlobal.rivet is undefined
in /wp-includes/js/dist/vendor/react-dom.min.js?ver=16.13.1
Uncaught TypeError: window.csGlobal.rivet is undefined
/wp-content/plugins/cornerstone/assets/js/app.bb9cc16.js
I am using the following versions of the X Theme and Cornerstone because when I upgraded to the latest version, the site became completely broken, so I had to recover from a backup:
Cornerstone: 6.08
X Theme: 9.0.8