PfSense does not boot without a monitor because of serial ports

Shuttle DS77u, DS67u and DS57u has problem booting without a monitor then upgrading from PfSense 2.3 to 2.4. These devices ran Legacy bios and not UEFI (UEFI is a new feature in PfSense 2.4) but never booted if the sceen was disconnected at power on, and never initialized the screen if plugged in after power on. This was because the serial ports were enabled in bios.

Bleupomme on PfSense Forum had the answer.

This just happened to me too on a shuttle XPC:
– Updated the BIOS, but this did not solve the problem
– Brute forced BIOS settings
-> by disabling the Serial ports the computer boots normally

Does pfsense 2.4 default to Serial and wait for some sort of connection when there is no monitor?