cart_order gets lost in session

When performing transactions (in this case with Rabobank) the cart_order variable might get lost in the session.
This leads to a warning showing up on the screen of the customer, claiming there was a problem and he should try again.
However the payment is handled correctly and the customer even receives an order confirmation in his mailbox.

The loss happens some time during the transaction itself: cart_order is still in the session when you get redirected to the bank and is gone when you return to the website.
This does not always occur, but unfortunately most of the time it does.

I've tried postponing the redirect to make sure that all variables are sent to Rabobank, but that didn't resolve the problem.

PATCH: reassigning the cart_order variable by giving it the value of ideal_lite_transreq_data['orderid'] (both originate from order-id anyways)