This is likely a server issue or being caused by another plugin.
After you make a category choice, let say “Deutschland”, you should end up on page with this address:
What is happening is your site is reporting:
Status Code:301 Moved Permanently
And then redirecting pack to:
When the redirect happens the query string is being stripped off and it is required to the list will be properly filtered.
With that said, I would suggest try deactivating any new plugins or plugins you upgraded since then to see if that resolves the issue.
Let me know.