I'm sniffing the XML using TCPTrace and fixing up calls as I go. I've managed to get a connection established to our web server. So far, so good.
But now, I select an item off the Excel menu that might actually try to retrieve some data. The web server answers two requests with what looks like reasonable XML, and the add-in tells me that I don't have a valid connection.
Crud. Now I need to talk to one of the writers of the add-in to find out what the black box is up to.