The Asus P8Z68-V Pro/Gen3 mainboard has an onboard graphics adapter.
When I attached a screen to the onboard adapter (which I usally don't use), I noticed that the screen was actually not staying blank, but Debian used the internal graphics to display a boot error - even when I set "Initiate Graphic Adapter" to PCIE/PCI instead of iGPU (internal).
I get two different errors, which also seems to be kind of random. Mostly I get this error:
Loading, please wait...
INIT: version 2.88 booting
[info] Using makefile-style concurrent boot in runlevel S.
[ ok ] Starting the hotplug events dispatcher: udevd.
[ ok ] Synthesizing the initial hotplug events...done.
[....] Waiting for /dev to be fully populated...[ 3.964436] usb-6-1.7: firmware: agent aborted loading ath3k-1fw (not found?)
[ 3.964561] Bluetooth: Error loading firmware
And sometimes it already freezes at
Waiting for /dev to be fully populated.
I tried to disable the onboard bluetooth adapter, but I still got the black screen error from time to time.
Next I saw that the Bios allows to run both internal and external graphics together - the related setting is
Advanced/System Agent Configuration/iGPU Multi-Monitor.
I disabled that setting. Now I don't get the blank screen error anymore.