Hey,
It was SagePay themselves that told me about it. Here is the email they sent us:
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Switching 3d secure off from the website or sagepay will not fix this issue. It is the field that the website is passing in the transaction registration post, to the gateway that is causing the invalid response.
The apply3dsecure field has to either be removed from the post, or amended and the white space after the 0 removed.
Please see below the string that we are receiving. Apply3dsecure is at the end of the post.
VendorData=42228&ReferrerID=CC923B06-40D5-4713-85C1-700D690550BF&Amount=390&CustomerName=xxxxxx xxxxxx&[email protected]&BillingSurname=xxxxxx&BillingFirstnames=xxxxxx&BillingAddress1=xxxx xxxx&BillingAddress2=xx xxxxxxx&BillingCity=xxxxxxx&BillingState=&BillingPostCode=xxx xxx&BillingCountry=GB&BillingPhone=&DeliverySurname=xxxxxx&DeliveryFirstnames=xxxxxx&DeliveryAddress1=xxxxxxxxx xx&DeliveryAddress2=xx xxx&DeliveryCity=xxxx&DeliveryState=&DeliveryPostCode=xxx xxx&DeliveryCountry=GB&DeliveryPhone=&FailureURL=https://www.bvt.org.uk/?page=gf_sagepay_form_ipn&SuccessURL=https://www.bvt.org.uk/?page=gf_sagepay_form_ipn&Description=Order #162631&Currency=GBP&VendorTxCode=162631-75cc4f5327bdb81c7758331af40d5a99&[email protected]&SendEMail=2&Apply3DSecure=0