Thanks for posting the detailed information.
That particular symptom is a result of the module using a slightly outdated method of passing error messages.
I've updated the file to fix that specific syntax problem.
http://www.zen-cart.com/downloads.php?do=file&id=387
I've not addressed any of the functionality of the module or how it works with the gateway. You'll have to sort that out with the sage/net1 folks who can help you by looking at whatever transaction details they're receiving from your store. But you'll have to call them for help with that.