My personal experience is that sometimes this work around can fix the problem, follow all steps!
• Uninstall the app
• Turn off WiFi
• Reinstall the app
• Ensure Auto Discover is set
• Enter sign in and password info
Also, double check that you are using your firstname.lastname@example.org as your sign in address and check that your password is entering correctly as it is easy to mistype on a mobile keyboard.
Update: The Mobility Service is not supported on dual-homed Front End Servers that are collocated with the Mediation Server role.