This appears to be a problem still with machines where unity-2d is installed but unity-3d isn't. I fixed it by sudo apt-get remove lightdm, and sudo apt-get install ubuntu-desktop (which reinstalled both packages, as well as unity-greeter... my guess is that just installing unity-greeter would have solved the issue).
This appears to be a problem still with machines where unity-2d is installed but unity-3d isn't. I fixed it by sudo apt-get remove lightdm, and sudo apt-get install ubuntu-desktop (which reinstalled both packages, as well as unity-greeter... my guess is that just installing unity-greeter would have solved the issue).