Did you update the Cornerstone to the latest version as well? If not, please update it, you can do this easily after you updated the X theme to the latest version.
Navigate to Plugins panel deactivate and delete the Cornerstone, then navigate to X > Validation panel, wait for a few seconds and X will re-install a compatible version of Cornerstone.
Remember to clear all your caching features (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
If that did not work still, please provide us login credentials (wp-admin and FTP) in a secure note so we can take a closer look.