Confirm Order Issue

Hi all i am using this plugin, it has been working fine, but now having some difficulties. When you go through the checkout process after you click submit confirm order on the last step you get the error below. The payment gets sent to paypal, but on the order screen you are only shown that an order has been placed for 0 products.

Thanks for your help!!

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [no address given] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.