I've had a conversation with IA Support about this error recently and they mentioned there are 3 current bug tickets related to this error. They suggested looking for a workaround since one of those tickets date back to 7.7 days.
I've used SAXParser in previous 7.8 and 7.9 gateways without issues and since version 8, it just about fails every time the script runs in the Client but works in the Designer.
Nick's workaround is great.
I found this link yesterday for another work around: