Order Redirect -1 Message

Dear Support,
We have just started using your plugin but we are having an issue following successful payment with VivaWallet when the user returns to the website.
They see a completely blank page with just -1 in the top left hand corner.
You can see this for yourself if you try visiting the link to the page provided.
Regardless of whether you click it now or successfully check out and pay, you only ever see a blank page with -1.
The payment is taken but as the order is unconfirmed so the status is wrong and no confirmation emails goes out.
Can you help please.
Rob

The page I need help with: [log in to see the link]