You're using a very old version of X (4.0.4) and the validation from that version is no longer compatible with Apex.
For this case, you will need to uninstall Cornerstone and manually update X. Please see the manual update instruction at https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62
Right after the installation of X, you will be taken to the X > Validation page and visiting that page, X will install the latest compatible Cornerstone version automatically for you. Depending on your server's connection to our server, it might take some time. Please give it a couple of minutes to install Cornerstone.
I'd strongly recommend that you regularly update the software versions because important security and improvements are released and also avoid update issues like this.
Hope that helps.