Hi all,
I have fixed it in my version. We use Form to capture the ProTx/Sage Pay transaction so we changed the following line as per SagePay document (it's near the bottom of the file):
File: \squirrelcart\payment_gateways\protx.php
Change:
Code:
$url = $gateway['Test_Mode'] ? 'https://ukvpstest.protx.com/vspgateway/service/vspform-register.vsp' : 'https://ukvps.protx.com/vspgateway/service/vspform-register.vsp';
to:
Code:
$url = $gateway['Test_Mode'] ? 'https://test.sagepay.com/gateway/service/vspform-register.vsp' : 'https://live.sagepay.com/gateway/service/vspform-register.vsp';
But remember this is for Form transactions only not Direct or Server as they have changes that need implementing too.
Be advised - don't muck around in these files unless you know what you are doing 
Justin