Actually this should have been a question, but meanwhile I found some root cause, so... maybe it helps someone else who has the same problem:
I have inherited a cDAQ-9135 with Windows operating system and I would like to enter BIOS.
The miniDisplayPort is connected to a standard monitor via a miniDP to HDMI/DVI/VGA converter.
The problem was that I got a screen output only at VGA, and only after the boot process had finished and the Windows screen was visible.
If I connect the cDAQ directly with a miniDP to DP cable to the same monitor, I can see the startup screen and everything from the beginning.
So there seems to be an issue with either my video converter, or with video converters in general.