Why does my checkout process keep looping back to the cart?

Store Design
1 
Hi folks, 
my checkout doesnt work properly. 
Somehow I always end up in "CART" when I click on "Auschecken" (which means "checkout"). 
It works fine in developer mode and it end up as planed at the next checkout stage where I am suppose to enter my info and credit card number, but it doesn't work in normal live mode. 
Its like being in a loop? What could that be?
Any help is appreciated. 

Thank you
 
URL: https://asphaltsurf-skateboardschule.myshopify.com/
Password: test1234
Theme: Dawn

![This is where the bug / loop is.|2880x1432](upload://eWYllHJLM5epkA8sO1rn6bflsBq.jpeg)
2

Hi @asphaltsurfer ,

I checked and the link you sent is not the site of the screenshot.

Can you send me the preview link? I will check it.