Avoid hang of headless Zotac server on Ubuntu Linux reboot. (How to disable the graphical console for grub and the Linux kernel.)

My Zotac server did not reboot without a monitor attached. Booting from power-up did work ok. The graphical console of grub seemed to be the problem. Since I do not use the monitor output at all not setting any graphics mode at all and using the 80x25 default console is fine for me. This is how to disable the graphical grub console:

Edit /etc/default/grub

Uncomment this line:
GRUB_TERMINAL=console

In addition I disabled setting any graphics mode on the Linux kernel and setting the timeout to 1 second.

My /etc/default/grub  file now looks like this:

GRUB_DEFAULT=0

GRUB_HIDDEN_TIMEOUT_QUIET=true

GRUB_TIMEOUT=1

GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`

GRUB_CMDLINE_LINUX_DEFAULT="text nomodeset"

GRUB_CMDLINE_LINUX=""

GRUB_TERMINAL=console

Reboot time: The time between pressing enter after 'reboot' and being logged in again with ssh is 25s.