If that is still randomly happening with the latest iOS version 10.2.1465 , could possibly then be eg: the app database might have an error in it that was “possibly carried over from the previous database” when it upgraded.
in which case you might need to “remove and then readd your account” either via the QR Code import option if you use eM Client for desktop or manually add it again if you don’t use the desktop.
You could also export the app settings to a .XML file via the Settings / Import Export options. Then uninstall the app and reinstall and then skip the initial setup and then reimport the settings . XML