I've had this same thing happen. I fixed it by opening the zen_paypal table in the database and changing the field 'order_id' to 'zen_order_id' which is what the error message seems to be looking for.

Someone smarter than me perhaps knows why this is happening. I have no idea. But I've taken this step and had no ill side effects. Still, you may want to backup, etc. first.

Hope that helps.

Matt