SagePay payment plugin - assistance required

Hi,

It can come from a lot of different things:
stackoverflow.com/questions/38480452/sag...saction-registration
It could be that you modified the settings of the custom address fields in a wrong way.
Or that some of the settings of the sagepay payment method are wrongly entered. Like for example if you've entered the credentials for the test mode but you've configured the payment method in live mode.
You should activate the debug setting of the payment method.
That way, you can look in the "payment log file" of the HikaShop configuration for the data sent to SagePay. Then, contact the SagePay support with that and with the error message you have and they can tell you what's wrong.