Ubuntu 16.04 hangs on shutdown/restart (Dell XPS 15 9560)

Eventually, the solution that fixed everything for me was the following (at least for my system configuration):

• edit /etc/default/grub

• add acpi_rev_override=1 to your GRUB_CMDLINE_LINUX_DEFAULT parameters

  (in my case the line looks like this):

     GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_rev_override=1"
  

• run sudo update-grub from the terminal

after shutting down your machine one more time from the power button, the problem should have been fixed.

I spent so much time on this, but eventually, the comment by Stefan was the solution for me.

As suggested, I switched to the proprietary NVIDIA driver, in my case NVIDIA binary driver- version 384.130 (proprietary, tested).