Hi There,
You are running an older version of X with a newer version of Cornerstone.
Lets try a manual update this time , please do the following :
1 - Revoke your purchase code from X Addons section (check the bottom of the page) and also remove it from your license page on Themeco.
https://theme.co/apex/licenses
2 - Temporary disable any caching plugins and purge all your server cache.
3 -Delete both X theme (/wp-content/themes/x) and Cornerstone plugin (/wp-content/plugins/cornerstone) by connecting to your FTP (your settings will be saved in the database)
https://theme.co/apex/dashboard
4 -Download the latest copy of X and upload it to your themes folder via FTP OR you can upload the x.zip via Appearance -> Themes section.
5- Latest version of Cornerstone will be installed automatically when you head over WP Admin > X section. Now you try validating your X theme again and then try to install any other extension and it should function properly.
Let us know how it goes.
Thanks!