Hi There,
Thanks for writing in! Probably you’re using an older version of X theme and Cornerstone plugin or your Contact 7 form is outdated.
You can delete the Cornerstone plugin folder through by connecting to your FTP (/wp-content/plugins/ cornerstone /). All the data will be saved in the database, so that you can safely delete the Cornerstone directory.
Then you should be able to login to your WordPress back-end. Check for outdated plugins or theme and update it first. You can refer to our version information listed here (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195).
Once you update everything, head over to X -> Validation section and wait for few seconds to install the latest version of Cornerstone plugin automatically. Also you can find the Cornerstone.zip file when you download the latest X theme from your Envato account under (x/framework/plugins/).
Thanks!