The model is a HP EliteDesk 800. It has one VGA and 2 DP. The screens display nothing (don't detect anything and turn off) when using DP only. When using VGA and one DP, the DP again displays nothing but the VGA goes black with the mouse as described earlier. The screens take DP natively, without any adapters.
If both DP ports in your system share the same issue I would:
- Try a different driver version, including the driver provided by HP*.
- Test with a different high quality DP cable.
- Test with a different DP monitor, if possible different model.
- Contact HP* and ask for possible known issues or BIOS fixes.