Checkout not working with more than one step

Trying to config a webshop. When I add more than one step in the checkout it is not working. If I got to checkout in the shop with more than one step the site gives a connection error. Tried different things (with and without registration) but there's always this connection error. Any idea how to solve that?

website: www.dokteranders.nl/dokteranders

It works fine for everyone else.
However, I see that you're not using the latest version. You should try to update. That might help.