Kubuntu doesn't work after upgrade to 16.04

I have a new System76 Lemur Laptop. It worked fine for a week. I installed Kubuntu, it worked fine. Then I upgraded to Kubuntu 16.04 LTS and the system doesn't boot fully anymore. The Kubuntu logo flashes but then never displays the login UI, I just see black. I can get a TTY by pressing CTRL+ALT+F1 key. I can run startx -- :1 from the TTY to get a bare bones graphical interface.

How can I get a usable graphical interface?

sudo dpkg-reconfigure sddm

and selecting sddm worked for me.

Run

sudo apt-get install --reinstall kubuntu-desktop plasma-desktop plasma-desktop-data sddm xorg xserver-xorg-core xserver-xorg-video-intel && sudo dpkg-reconfigure sddm

The 1st part will reinstall kubuntu's KDE plasma, the X11 display server, the sddm display manager, and the intel video driver; the next will allow you to set it as the default display manager. If some of those packages aren't installed, it will attempt to install them.

Logs are located at /var/log. Look for the log for Xorg and sddm.