There could be many possible causes, the first one is caused by CORS. You have configured your site admin to use secured URL (HTTPS) while the front is non-secured (HTTP). Cornerstone builder is mixed with both backend and front end, and this setup of yours triggers the CORS issue.
Since this is live, I can't disable your HTTPS plugin because it usually breaks a site when it's dependent on the plugin. These are my recommendations
1 Don't use plugin to enforce SSL/HTTPS on your site (it may skip internal URLs)
2 Configure your two URLs in
Admin > Settings > General for HTTP or HTTPS at the same time (to make sure backend and frontend uses HTTPS)
3 Install Better Search Replace plugin and replace this string
of course with your own domain name
Let's do that first to fix your site's CORS issue, I'm not saying this is the final cause/issue but we need to eliminate them one by one.
I also flushed your site's permalink and seems it fixed the CORS issue, but make sure to follow my recommendation to prevent future issues. Now, the error being display is 404 error and internal server error on both cornerstone endpoint. Please provide your FTP login credentials as well for further checking. Looks like it's trying to switch from current cornerstone endpoint to legacy cornerstone endpoint due to 404 error. Plus, if you host has security feature like
mod_security then please disable it first while we're investigating.