It seems to be the phone. Have you shut down and restarted the phone? Find the WiFi signals list and tell it to forget that connection then select it again and re-enter the password. You may also try to do a complete reset of the phone. Consult the factory website of maybe you can find it in the setup menu.
Another possibility is that your secure network may be on a different frequency (5 GHz vs 2.4 GHz) than the guest, try using the other band.