Tom,
We are noticing this problem with the latest iOS update. It is related to token expiration in previous releases of Explorer on iOS . It is currently being investigated and I will update this thread when more information is known.
To get around this issues please uninstall the app from your device and reinstall the latest version of Explorer for the Apple App store. This should not take too long as it is an iOS app and this will clear out your application settings.
Thanks for your patience,
-Brent