Sage Pay: Error 3085 : The FailureURL format is invalid.

Just a quick one.
If you ever receive this error when trying to put a purchase through via SagePay, dont spend hours wondering whats wrong with your FailureURL. Its nothing to do with that in most cases.
Double check that the Encryption Password you are using in your system is exactly the same as the one supplied by SagePay.
I have just spent an hour trying to debug my ZenCart module before checking the Encryption Password was correct.
Turned out the client typed in a lowercase “L” rather than an uppercase one – Doh!