[Kassa Compleet] What is the best way to report an issue in the official extention for Magento 1.x?

I have got a response from Kassa Compleet:

The best way to report your problem with the Kassa Compleet plugin for Magento is to send in your problem to this email-address.
Preferably well documentated with screenprints.
We will get it at the right place to get a solution.