Thanks for updating the thread.
I cleared site cache and Cornerstone is working fine and I am able to edit pages but the loading is definitely slow. In an another instance I see continuous Cornerstone loading icon on your website and in dev tools I see
Preview timeout error message. So there's issue on on the website.
Mostly I have seen that
Preview timeout error is because of caching. First, please clear cache and then disable WP Super Cache plugin and try to load Cornerstone again. See how it goes.
I am not allowed to do deep troubleshooting on your website as I am not allowed to do so without taking due permission from customer. Please try the solution I have shared above and if Cornerstone is not working then please give us the permission to test things further. We may have to perform some tests on the website which may affect the performance, appearance, layout and functioning of website temporarily.