Situation: I have upgraded two Nvidia Optimus laptops from Linux Mint 17.3 to version 18.
Problem: After upgrade, Nvidia Optimus does not work, both laptops booted into Cinnamon fallback mode.
Temporary solution: I changed the graphics driver to Nouveau, then the Cinnamon works, but there is an "X" icon in system tray indicating Active graphics card: Unknown. And I obviously need a "real" solution.