Hi Kevin,
and thank you very much for your response.
As the problem occours before a operating system is loaded, a Windows dxdiag report would be worthless. Hans described it:
After saving BIOS/reboot, no video anymore......monitor keeps in sleep.
...
Next reboot (still no video), I did (blindly hit F2 a few times, then F9 + Enter and F10 + Enter.
Magically, after the reboot, my BIOS splash screen appeared.
What he was saying is, that neither boot screen nor Intels Visual Bios would show up on the Monitor after flagging the processor graphics as primary video adapter in UEFI. The monitor would get switched in DPMS-Standby on reboot. The strange thing is that aside from not feeding a video signal to the monitor, the computer works normally. Operating systems boot and would behave just normal. But you have no chance using the computer as the monitor is black and in standby. VNC and Teamviewer to the machine confirm this.
So, this problem resists in UEFI or BIOS firmware, at least on DH87RL. Please forward it to UEFI firmware engineers as they would be the right guys to solve this.