One of our users is unable to log into Field Maps; she's using iPhone with the latest version of the iOS and has the most up-to-date version of the application.
The map is served through ArcGIS Online. When she selects the option to log in, the app comes back with the message 'Sign In Error - Not logged in' (see the image below).

She's already tried uninstalling and reinstalling the application, but is still getting the same message.
Is there anyway of getting around this issue?