Why won't my checkout load properly in test mode?

[MrSchleicher]

Hi everybody,

I have the following Problem and would very appreciate quick help. 

If I try to checkout in the rest mode though the Onlineshop modifier I get this error:

And if I try to checkout by looking up the webpage like a normal customer would I can't even hit the Pay Now button:

When I tryed to checkout before activating the test mode it gave me a loading error with Page not found. 

What can I do to fix this problem? 
Is it maybe related to a domain issue as I have a custom domain and the Checkout still relates to the old not existing one? 

Thanks for any help in advanced