No confirmation on checkout

[imagoel]

Hello,
I have a danish webshop www.kagekaelling.dk
When people go to my store, they can add products to the basket and they can go to the checkout.
Afterwards they are ment to chose shipment and payment methode, wich is fine.
When they are to confirm the order, the webshop just says "Error 500".
I do not any longer get the order on my email - and my customers do not get an confirmation email either. (The one that says "order has been placed").
When i open virtuemart and go to "orders" on the control panel, I do see the order, even tho I or the custormers never recieved the email.
However - when I try to mark as cancelled, shipped or any other, it goes back to the "Error 500".
How do I solve this?

[Milbo]

joomla version, virtuemart version, what happens without SEF?