Hello @Hannes,
Thank you for using our theme. You are using 8.3.1 version of the theme, it is not the latest one. We would recommend updating first – https://xstore.helpscoutdocs.com/article/63-theme-update (for the theme) https://xstore.helpscoutdocs.com/article/37-plugins-update (update XStore Core plugin also).
Do not forget to create a backup before any update, as described in docs. After the update, clear cache and check if variation can be selected.
Let us know the result
Regards