I am using Magento 1.4.2 and customers encounter the error screen below when they try to checkout from the secure checkout area using the Amazon Simply Pay option. I need this error message to be fixed. Amazon payment buttons seems to work fine when checking out from the shopping cart.
Screenshot: [url removed, login to view]
Input parameter signatureVersion = null is invalid. This signature version is no longer supported. Please use version 2 to sign requests.
For further information on error codes please see the Integration Guide.