OK. It seems I managed to resolve it. I had to change the plugin's controller: PaymentWorldPayController.cs ....
Hi Aniklaus,
Many thanks for posting this! It's also fixed my nopCommerce 2.50 instance. I'm surprised this fix hasn't made it into the final version of the extension as it seems to be required for WorldPay payments to work. Even the 3.10 WorldPay plugin doesn't work :(
Is this change working well for you on your live site? Please let me know you've had any issues with this fix.
Nick