Thank you for writing in, you updated the Cornerstone to the latest version (3.1.6), but you leave the X behind. Please update X to the latest version (6.1.6) as well to avoid incompatibility issues.
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.
Let us know how it goes,