I just updated Ubuntu 18.04 (just the regular "install pending updates" when i shut down). Now I booted my pc, and was greeted with a debian Login screen. This makes absolutely no sense... Is there a fix, or do i have to revert to previous version? Date of installing the update is 8/23/2018.
I had the same problem. By looking at
/var/log/apt/history.log I could see the update had removed ubuntu-session, ubuntu-desktop, gdm3 and gnome-shell.
After reinstalling those, it works fine again (maybe the most recent updates of the Linux kernel to 4.15.0-33 helped too).