Debian Stretch doesn't boot without Monitor

[Wolfgang <lister@feedyourhead.at>]

Hi,

I have an embedded device(/small pc) and I want to run Debian Stretch on
it. But I am experiencing a strange problem: it doesn't boot without
monitor. I created the following cronjob to verify if its
up'n'running(or not):

@reboot root beep -f 300.7 -r 2 -d 100 -l 400

My first guess was: some weird BIOS-Setting. But I didn't find any weird
BIOS-setting. So I tried to run a different OS, and it booted up correctly.

I also tried to use the Debian-Stretch-Disk on another hardware and it
boots even without a monitor. The problem seems to be a related with my
hardware.

The device has intel-graphics: i915

I installed(without success) the following firmware/microcode:
firmware-amd-graphics
firmware-linux
firmware-linux-free
firmware-linux-nonfree
firmware-misc-nonfree
amd64-microcode
intel-microcode

Does anyone have a clue?

Thanks in advance
Wolfgang