When I first installed the Android app on a Kindle Fire (running CyanogenMod 10.1), it kept self-closing without any error message in my face.
Turns out that the reason is that I was typing in my email address instead of the username — this was mostly caused by the fact that the proposed input editor (SwiftKey) was set up in a way to accept an email address (the @ on its own key with the default domain completions) rather than the generic one.
A better way to tell the user that the login is invalid would be nice.