Hi Andreas,
I tried to replicate your setup on my development area with the latest version of the X theme and cornerstone plugin then I don’t see any issue like yours. I suggest that you deactivate all your plugins except for the cornerstone plugin, WordPress store locator, and Redux plugin. If the issue is resolved then activate the plugin one by one to find the culprit.
On the other hand, if the issue still persists after the first troubleshooting, maybe your X theme is corrupted when you upgrade it to the latest version. I suggest that you reinstall it by deleting the X theme and cornerstone plugin then go to your themeco dashboard and download the latest version. Once you are done downloading the files, upload them to your staging area and install the cornerstone plugin.
Hope that helps and let us know how it goes.
Thank you.