Critical Error on Website when checking out using SagePay Form

Hi there,

We purchased the SagePay Form Gateway plugin a couple of years ago from the Woocommerce website and it’s been working fine until recently.

A customer called up yesterday to say that they have checked out and opted to use SagePay and when it goes through to the page it says the following:

Thank you for your order, please click the button below to pay with SagePay.

There has been a critical error on your website.

Can anybody help as this is quite a serious issue if customers are unable to checkout?

Thanks in advance.

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