Paymate Opencart 2 Transaction Failed Message after change from HTTP to HTTPS

I need some advice please. Yesterday, we upgraded our site from http to https and everything appeared to go well. Today we have discovered a problem when processing credit card transaction using Paymate. The transaction appear to process as normal through the shopping cart and checkout process. The customer selects the "Pay by Credit card" option during this process and completes the checkou at which time, as normal the customer is taken to the Paymate site so they can enter their credit card details. Again, as usual, some of the fields (customers name, address, emai, phone transaction value) are automatically populated on the Paymate site from information parsed from our site via the Paymate extension. The customer enters their credit card number and completes the transaction at which time, they hit the "Return To Merchant" button. Now normally, they would then be taken back to our site where their shopping cart would be empty and a thank you message displayed. What is happening now is, they are taken back to our site, where a "Transaction Failed" message is displayed and their shopping cart still has the items in it that were there previously. The actual credit card transaction does get processed as usual though. The customer does get charged but the cart does not get emptied. I have looked through the Forums and have tried making the recommended changes such as changing url->link('checkout/success'), to url->link('checkout/success', '', 'SSL'), but it has not worked. Any advice would be appreciated.