Hmmm. You are using the latest ecommerce from Jedatech?
I get a similar error when I undo his changes in the OrderForm class. Around line 200 in the processOrder function should read:
// Save payment data from form and process payment
$payment->OrderID = $order->ID;
$payment->Amount->Amount = $order->Total();
$payment->Amount->Currency = Order::Currency();
This handles the changes made in the Payment module to implement the Money class.
Hope this is of some help. Otherwise, do you have a stack trace?