Hi There,
In that case we reccomend you a manual update.
Please do the following:
1. Revoke your License from X Addons section (check the bottom of the page) and also remove it from your license page on Themeco.
https://community.theme.co/my-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)
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 X Addons 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!
Joao