I am using an iPhone 7 with version 6.7.0 of the official geocaching app. I have tried uninstalling and reinstalling the app, using different hotspots, and even turning Wi-Fi off all together but nothing so far has worked. I still keep getting the message that my username does not exist, but I can log in from either my computer or my tablet with the same username the only device I can't log in from us my phone.