5080 : Form transaction registration failed.

I am using Gravity Forms 1.2.1 and SagePay Form Gateway for Gravity Forms 1.1.2 with WordPress 4.7.

I am managing two separate WordPress installations which live on the same domain. The first instance has been operational and integrated with Gravity/SagePay for a couple of years. Recently there was an attempt to set up a very similar form on the second instance.

This second (newer) instance was set up with the exact same SagePay information as the first instance. For example, it is configured in live mode and with the same encryption key (etc).

I’ve since taken over maintenance and am debugging an issue that is happening consistently. Even given the simplest form possible (first name, last name, email, address, hidden product), once redirected to SayPay there is a 5080 : Form transaction registration failed. The form submits fine, but the error is thrown by SagePay on page load and there is no way of progressing. It happens every time.

My understanding is that this is a generic error from SagePay, so the code itself doesn’t help in tracking the issue down.

Please let me know if you are aware of anything that may cause this. Failing that, please let me know how I can approach debugging the pre-encrypted request body.

Any help appreciated. Thank you.