Upon checking, I see the reason is that you have missed some major updates and now you're using an older version of X theme (v4.4.2) and Cornerstone plugin (v1.2.4).
The latest version of X theme is (v5.2.5) and Cornerstone plugin (v2.1.7). So at this point, the best possible option would be to update your X theme manually for once using this guide (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62). Then disable/delete your Cornerstone plugin and head over to X -> Overview section and wait for few seconds to install the latest version of Cornerstone plugin automatically.
Then you should be able to re-validate your site and use automatic updates here onwards.
Please note: Since you're using an older version now, we strongly suggest you to take a full backup of your site before updating.
Hope that helps.