Ubuntu 16.04 ++ forces logout when trying to shutdown or reboot from menu – guilty party: PLANK dock

Source: http://www.linuxslaves.com/2016/06/fix-cant-shutdown-from-ubuntus-toppanel-after-install-plank.html

Shutdown or Restart – forces Logout, then from main Lightdm screen shutdown/restart works.

Problem is with PLANK dock.

Open: Startup Applications
Select: plank; for the ‘command’ enter:

sh -c "sleep 10 && plank"

All will be OK on next login/restart.