I get this error while trying to test the latest version (3.0) of Sagepay with the the form kit. But when yo login to Sagepay simulator, it will ask you to change the sagepay.api.formPaymentUrl.TEST key in the web.config file to https://test.sagepay.com/Simulator/VSPFormGateway.asp. But then you get this error "The VPSProtocol field you supplied contains invalid data. It should contain the value 2.23". So I changed sagepay.api.protocolVersion key in web config to 2.23. After doing this I shopping cart fails on ExtraInformation.aspx page and there is no button to proceed further. After spending 2 days on Google trying to find a solution I finally gave up and followed a different approach. I tried debugging the code.
In web.config file, have the following:
sagepay.kit.basketxml.disable = true