What could prevent an Ubuntu server from booting without a VGA connected monitor?

Solution 1:

Make a backup of /etc/default/grub and then edit the file with ...

sudo nano /etc/default/grub

and then find the line ...

GRUB_CMDLINE_LINUX_DEFAULT

and remove quiet and splash if they are there (doubt that they are cuz otherwise it would never work) and add ...

nomodeset

From comments for you this would be

  GRUB_CMDLINE_LINUX_DEFAULT="biosdevname=0 nomodeset"

You will need to do a

sudo update-grub

to reload grub. A reboot should let the system pass grub without a monitor.

Solution 2:

This is quite likely a hardware issue and not related to the OS

Many older motherboards, especially non-server ones require a connected monitor on boot during the P.O.S.T sequence otherwise the motherboard will halt during the boot sequence never reaching the boot loader.

This often unfortunately holds true even if you enable the Don’t halt on Error option in the Bios.

Since a small amount of current is sent through the pins to check for connectivity you can make a dummy connector by simply connecting three 50-100O Ohm resistors between the pins shown below

That's how I solved this issue with a "server" I had ;)