Actually, I'm suggesting that you modify this:
DataConnections["CustomListItemsSubmit"].Execute();
Create a web service connection instead and when you execute it you can pass in the output nav and errors nav to return any error info and track down the issue.
Edited to add: Like this:
WebServiceConnection shpSubmit = (WebServiceConnection) this.DataConnections["CustomListItemsSubmit"];
Then shpSubmit.Execute(null, outputnav, errorsnav);