When I login to Kubuntu (16.04.2 LTS), kde-plasma desktop loads, but then it will go black with a white line at the bottom (probably the taskbar) and the desktop becomes unusable, so I must switch to a tty and run `killall kwin_x11` to make it work.
I'm sorry, but the version included in Kubuntu 16.04 is no longer maintained by us. This means we cannot properly investigate. It is also possible that the problem was already fixed. Please consider upgrading to a newer release, e.g. our LTS release version 5.8.