http://support.microsoft.com/kb/2636318

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 alias@domain.com 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.

http://technet.microsoft.com/en-us/library/hh690037.aspx 

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