Fix issues with Joomla Opencart (mijoshop)

The problem seems to be for customers that already have accounts. So for a new customer, selecting a product and going through the checkout process it seems ok. Also if an existing customer logs in, selects product and checks straight out it's usually ok too, but if an existing customer logs in, adds product, then perhaps stops checkout half way through to add new product or modify or check their account details and then try to checkout again it triggers a error. Error not always the and not always at the same point. Very frustrating. Here is a video of an error being generated: [login to view URL]