You updated the X (6.0.4), but your Cornerstone is still in old version. Please update the Cornerstone to the latest version (3.0.4) as well.
You can do this by deactivating and deleting your current Cornerstone. Then navigate to X on your dashboard and X will automatically re-install the Cornerstone in the latest version.
Remember to clear all caches (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.
Hope that helps,