"Unable to convert Webstore XML to ShipRush TRequest XML"

Hi,
We sell on eBay and Amazon, and we also have a custom e-commerce platform. We do all of our shipping via stamps.com.

I found sample xml xml output for the ZenCart module so I built an interface to export our order data that mimics the ZenCart data feed.

Everything seems to work fine initially. And when I first tried it worked fine for a few days. But eventually when I try to export I get the following error:

"Unable to convert Webstore XML to ShipRush TRequest XML. Object reference not set to an instance of an object."

Can anyone shed any light on what this means?

Thanks for your help.