I have made a fresh install of Kubuntu 16.04 via USB stick. The sha256 checksum of the USB stick matched the one on the download page. All installed packets have their most recent version. I have a AMD A10-7850K Radeon R7, 12 Compute Cores 4C+8G so I don't have a separate graphical card. On Kubuntu 14.04 I use the flgrx driver but this is no longer available on 16.06.
After I entered my password the blue progress bar reaches its end, but nothing happens anymore for 30 seconds. Then I get a pop-up with following message: "Plasma can not start. All shell packets are missing. This is related to the installation, please take contact with your distribution." (translated myself from Dutch). Then I get a black screen.
When pressing Ctrl + Alt + Delete I get the option to log out or to reboot.
Note: I have seen posts of other people having the same problem on other forums