Hello! Do you work on lbComm1 App?
If yes, you binded handler for "backbutton" on the first screen. But you go out from this screen with non-ajax call function naviagateTo('screen_name'). Because of this your first screen is removed from DOM and handler for "backbutton" is removed too. In order to prevent this behavour you should to use ajax call function naviagateTo('screen_name',{}), or if you don't want to use ajax navigation you should to bind hadler for "backbutton" event on all screens. Also remove JS (where you check registration) from onLoad event and put it on Page Show event.