Order Completion Error

We're getting an error message even though the order seems to be going through (confirmed via Paypal that the payments have been processed on these test sales) though I'm not receiving an order confirmation email nor does it redirect to an order completion page as it should.

We've disabled ALL plugins other than Marketpress and it still occurs so that should rule out any conflicts with 3rd party plugins, correct? Additionally, the items remain in the shopping cart after the error message is rec'd & the store doesn't show a sale.

Please advise at your earliest convenience, we';d really like to get these issues resolved ASAP since we've been setback over a month already with numerous issues and have a launch date of 11/29...