I installed ADS PRO - https://adspro.scripteo.info/ plugin to manage my banner ads.
I inserted one of their shortcodes in one of my headers, and it ended up breaking my editor. I literally could not edit any of my Cornerstone pages. I was getting this error:
The preview could not load. This is most often related to a plugin conflict or aggressive page cacheing. Checking the developer console for errors could indicate what went wrong.
When I deactivated the plugin, my editor worked again. Just wanted to post this to help others who might search for this in the future.