Thanks for asking.
Can you please try to completely deactivate and delete Cornerstone from Plugins > Installed Plugins. After that please login to FTP using softwares like FileZilla and under /wp-content/plugins/ directory make sure to delete Cornerstone folder.
Next, please visit X > Validation and doing so will automatically install most recent version of Cornerstone and see how it goes, wether or not doing so resolves the error message you are getting.
In case issues is not resolved, please try increasing memory limit. Click here for more detailed information and how to increase the PHP memory limit.
If you still have problems kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard along with FTP login details using the Secure Note functionality of the post to follow up the case.