Your version of X (v4.6.4) and Cornerstone (2.1.7) is not compatible. Please update your X theme to the latest version (5.2.5)
Updating Your Themes and Plugins
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 it helps,