Hi there Markus!
Don here from Shopify. :)
Third-party apps are actually supported by their developers, rather than by us here at Shopify.
Not to worry, as you can simply reach out to the developers directly so they can help troubleshoot this for you.
I wasn't able to find an entry for that app in our app store though it does look like this is their website so you can try to find out how to contact them from there.
If there's anything else in terms of your core Shopify admin we can help out with you can get in touch with us anytime, 24/7 via our support site. :)
All the best!