Hello Maik,
Thanks for posting in! This issue might have happen because you forgot to update Cornerstone. Could you please update to the latest versions?
Our latest versions are:
- X theme
7.2.3
- Cornerstone
4.2.3
This latest release contains fixes for several issues so be sure to check out the changelog (http://theme.co/changelog/).
After doing the updates, always remember to clear all caches (if you are using WP Rocket, WP SuperCache or W3 Total Cache) when updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
If nothing is helping, provide us access to your site so that we can check your settings. Please create a secure note with the following info:
โ Link to your site
โ WordPress Admin username / password
โ FTP hostname
โ FTP user
โ FTP password
โ
To know how to create a secure note, please check this out: https://theme.co/docs/getting-support

Please let us know how it goes.