Let's see. I would try to test just the list submit. I'd add the batch node that you are submitting to the form so you can make sure it is populated, and add a button to just execute the data connection... maybe see if that reveals anything? Anytime I've run into this, loopback check is disabled on the server and I get an error in the browser, so I'm a little stumped as to where to begin. :)
Hilary Stoupa
