Hi There @motopassmt,
Thanks for writing in! You need to update Cornerstone plugin as well to the latest version (v3.5.5) in order to make it compatible with the latest version of X theme.
Once you download the latest version of X theme from your Themeforest account, you can locate the Cornerstone.zip plugin file under the following location /x/framework/cornerstone.zip.
You can simply disable and delete the existing version of your Cornerstone plugin and then install the latest version. All the current settings will be saved in the database, so you’re safe to delete the existing Cornerstone plugin.
Once you’re fully updated, please make sure to clear all caches by following this guide (https://www.wpbeginner.com/beginners-guide/how-to-clear-your-cache-in-wordpress/) and re-test your site again.
Let us know how it goes.
Thanks!