I found the following work round (from various sources) which works for me:
Edit file /etc/modules: add line "apm" (no quotation marks)
Edit file /etc/default/grub: at GRUB_CMDLINE_LINUX_DEFAULT add "apm=power-off" in quotation marks and directly after any other kernel line commands without leaving a space between them (if I left a space it didn't work!)
Then in terminal type: sudo update-grub
Maybe this is a bug in ACPI.
I found the following work round (from various sources) which works for me:
Edit file /etc/modules: add line "apm" (no quotation marks) LINUX_DEFAULT add "apm=power-off" in quotation marks and directly after any other kernel line commands without leaving a space between them (if I left a space it didn't work!)
Edit file /etc/default/grub: at GRUB_CMDLINE_
Then in terminal type: sudo update-grub
After rebooting shutdown works normally.