To have your Back button working again, you need to disable Joystick Support on Kodi
Add-ons -> My Add-ons -> Peripheral Libraries -> Joystick Support -> Disable
For this changes take effect, you need to terminate Kodi.
Press and Hold the Home Button to open the application switcher.
Press and Hold the ok button in order to terminate Kodi process.
Now, open Kodi again and make sure Josytick Support is disabled.
It should now be working.