There seems a bug in the DP module or BIOS firmware.
I have a ATEN CS1924 KVM switch which connects to the DP module.
The Display works, but as soon as I change to a different PC an back, the port seems to get into a strange state. The KVM only switches for a short time, beeps and switches back.
Unplugging the DP module, switching the the KVM and then inserting the DP module works.
Also plugging in the DP module, then the DP cable work.
There seems to be some problem with the handshakes. The KVM switch does some monitor simulation on the inputs not used, so the PC does not see a unplugging event.