Can't progress beyond shipping details page...

It was all working fine, no changes. All of a sudden the site won't go past the shipping details page. Clicking on the "Continue Checkout" button it should go to the paypal page but it is not.

I've upgraded wordpress and marketpress to the latest versions.

I've changed the theme to a default theme and the same thing happens.

When I click the "Continue Checkout" button instead of the paypal page loading I see a 500 server error instead of lots of paypal urls...

No paypal or hosting details have changed.

This is urgent as our customers cannot buy from our store, please help, we have also tried in multiple browsers and computers.