Dr. Nikolaus Klepp wrote:

> Remove or comment out all lines except these 2, then reboot:
> 
> auto lo
> iface lo inet loopback

good catch Dr. Klepp - I have completely forgotton that there are still
people on desktops with entries in the file.
When I read your question it lighted up again - to manage the networks by
NetworkManager there should be no entries in there (except lo as pointed by
you)