I've tried all your steps and it is still not working. I have updated Cornerstone to the latest version and the error message is now:
The preview was unresponsive after loading. This is most often related to a plugin conflict or aggressive page cacheing.
I have deactivated all plugins and cleared cache and deactivated cacheing plugin , and the error message still appears. I've also changed the theme to parent theme, reset htaccess file and also increased the php memory limit and still the error message persists. Please help!