I just had this same problem,
sudo was working fine until I clicked on the icon for "Wired network connection" and the manual configuration, then I edited the domain. This had upset sudo. When I reset back the domain (in a similar way) sudo started to work again.
I just had this same problem,
sudo was working fine until I clicked on the icon for "Wired network connection" and the manual configuration, then I edited the domain. This had upset sudo. When I reset back the domain (in a similar way) sudo started to work again.