Invoicing not working – normal GD Checkout comes up instead

Hello friends,

believe it or not that big website is finally online, you can see it at https://hochzeit-premium.de – as some of you know the development page was at http://boz1.timmeserver.de/ before. I copied the page with duplicator and the uploads files separately afterwards.

Mostly everything is working but I have a few little bugs.

First thing is: The invoice Plugin is not working. All settings are like on the testpage, I get no notice what could be wrong so I hope you guys can help me out.

What happens: When I try to add a listing I dont see the normal invoice page with all those infos. The checkout page should be https://www.hochzeit-premium.de/kasse

but instead I am landing at:
https://www.hochzeit-premium.de/gd-checkout/

Here is the link to add a listing in the beauty category for example:
https://www.hochzeit-premium.de/add-listing/?listing_type=gd_beauty

Can you please take a look, whats going wrong over there?
Do I need to add the new domain somewhere in the settings maybe?

Thanks in advance,
Atilla