Hey Sören,
We don’t have that issue in our test site and it’s unlikely to be caused by neither X nor Cornerstone but to be sure, please first switch to the parent theme then check if the issue persists. If it’s fixed, you might have custom code in your child theme that is causing the issue. You should consult with a third party developer for this case.
If the issue persists, please disable all optimizations in your Autoptimize plugin and clear all caches. Then, deactivate all third party plugins. Check again if the issue persists.
Also ensure you’ve installed X properly. The X folder should be under wp-content/themes. A common mistake is installing X in a sub-directory like wp-content/themes/x/x.
If you’ve installed X correctly and all third party plugins are deactivated and the parent theme is active, please give us WordPress Admin and FTP credentials in a Secure Note.
Thanks.