Unfortunely the credentials dont seem to be working.
Upon further investigation, you are a few versions behind on your X Theme.
At this time let´s perform a manual update.
Please backup your website and follow these steps:
- Revoke your License from Wp Admin | X Section and also remove it from your license page on Themeco.
Temporary disable any caching plugins and purge all your server cache.
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)
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.
Latest version of Cornerstone will be installed automatically when you head over X tab of your dashboard. 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.