Hello, @Oliver Theiß,
I have checked the same thing on our test site and it works fine already https://postimg.cc/p9H19fvZ The issue appears only on your site. I can see that you are not using the latest version of the theme on your site. Please update the theme and its core plugin “XSTORE CORE” to the latest version then check back to your issue after clearing the browser cache. You can follow this article to update the theme successfully on your site: https://xstore.helpscoutdocs.com/article/63-theme-update
And if the issue will still persist then please try to disable all the third-party plugins (including cache plugins), that do not come bundled with XStore https://xstore.helpscoutdocs.com/article/36-included-plugins, clear the cache of the browser, and check then. Maybe one of the additional plugins is the reason for the problem.
And if the issue will still persist then please remain all the third-party plugins deactivated on your site and allow permission to debug your site which includes deactivating all the third-party plugins and on-site Customization as well so that I will check and assist you accordingly.
Note: Please take the complete backup of your site first.
Regards 8Themes Team.