I’ve used Cornerstone with the X-Theme on several sites and have not run into this issue before. When certain plugins are activated, pages do not load to edit in Cornerstone. I’ve followed the instructions on this support ticket: https://theme.co/apex/forums/topic/cornerstone-not-loading-3/
I’ve installed Transient and did the SAMEORIGIN header, but neither of those fixes resolved it.
The plugins I am having issues with are Slider Revolution, Ninja Forms, and Google Analytics.
Below is a screenshot of my console when it fails.