Error at checkout page - HTTP 500

[sadiwadi]

My site url is www.trendee.dk (sorry, it's in danish)

 

I have problems at the checkout. when you've put something in the basket and typed your details in at the end and click go to checkout, i just get a page error.

 

I really need to fix this as soon as possible as i keep getting emails from my customers

Also i've tried going to backend and changing it to a five-step-process to see if that helped, but as soon as i press "save" it just goes to error page again.