Is there any chance you can test a different DP cable? Could you please let us know the driver version you are running? Please go to Device Manger> Display adapters> Intel(R) HD Graphics right click on Properties and then go to Driver, you will be able to see the version installed. The test you have done with the HDMI is indicating that the graphics portion of the processor is working, so at this stage the issue can be related to the cable or the port itself.
I have used the latest drivers but this can not be driver related since the NUC was given also no display when power it on so at that point there is not driver needed to display the Intel NUC POST screen.
Since i have changed to the HDMI cable the client has not called back (this since begin of july) so it was only at the display port.
Maybe the cable on both systems where i have had this issue the cables are from the same order .
But what i dont understand i on both systems that i have worked for a few days (+-5 days) and suddenly when starting up in the morning 'no display'
I will try again when i sell another NUC with another brand cable.
Dcaub, if you get the chance to test a different DP cable, update the thread, I would like to know about that. You can also take into consideration the previous input, it should give you a different standing in order to address possible issues like this one.
rado77, thank you for your suggestion.