WorldPay Callback Failure

[stuffedhippo]

I have a site which has been using WorldPay module (developed by Prestashop) version 4.1.1 running under PS version 1.6.1.2.

 

Suddenly every WorldPay order is being flagged as having callback failure:
 

 

Our systems have detected that your callback has failed.

This callback failure means we were unable to pass information to your server about the following transaction:

Transaction ID: [transaction ID]
Cart ID: [cart ID]
Installation ID: [install id]

Error reported: Callback to: https://www.sunrisetools.co.uk/modules/worldpay/validation.php: failed CAUSED BY Received fatal alert: internal_error

 

The call back validation page is correct within WorldPay admin and the files exists on the site.

 

The customers payment is taken but they are then returned to their Order History page. The order does not move from the Shopping Carts into a validated order. My client then has to manually create an order from the cart.

I have messaged Prestashop who developed the module but had no response. Considering this is an expensive module it isn't too good!

 

I have checked the WorldPay configuration and everything is OK.