Magento SSL Sagepay empty cart issue

Question

Trying to setup SSL on the Magento checkout pages but have run into this issue:

After payment details are submitted, the payment goes through fine (checked with sagepay) but when sagepay redirect to the success page (/sagepay/server/success/?SID=session_id), it then tries to redirect to /checkout/onepage/success/, where I get an error saying my cart is empty.

This only happens when SSL is enabled, if I turn off secure urls (in the magento backend), everything works fine.

I'm using the Pod1_SagePay extension, in the iframe mode.

Any idea where it might go wrong?