Problems with checkout

[LAMF]

HI,
I have a client that i have VM v.2.0.20b installed on.
Now all of a sudden when a customer places an order and comes to the checkout page where you fill in all your details and either register as a customer or checking out as a guest.
When clicking for the checkout a 404 page is displayed.
I don't find anywhere anything related to this in the Joomla and VM backend.
Please help and guide me someone.
The URL is www.stigsbergsgard.se (in swedish)
Thanks.
Micke