Displayport-HDMI cable strange behaviour

[madahun]

So i got a new pc along these lines:
Kontron KTQM87(its a mobo with an integrated intel i7 4860EQ and an iris pro 5200)
8 gigs of hyperx blue 1600mhz ram
Bitfenix whisper 450w
and i already have a LG 24MP55.

My motherboard has 3 Displayport 1.2 outputs. I red the manual which stated that it supports a passive Displayport-HDMI adatper so i got a maleDP-maleHDMI cabel.(My monitor has 1 HDMI and 1 DSUB port).
When i connect my pc to the monitor i get no signal. The HDMI port on my monitor works, it's hooked up to my older pc right now. The curious part is that if i use a HDMI-DVI adapter on the HDMI end of the cabel and connect it to my brother's monitor using the DVI end everything works fine.
Sadly i dont have a femaleDVI-maleDSUB adapter on hand. I restarted everything numerous times and checked if all the cabels are pluged in correctly.

TLDR: DP port on monitor->DP-HDMI cabel->HDMI port on monitor= NO SIGNAL
DP port on monitor->DP-HDMI cabel->HDMI-DVI adapter->DVI port on monitor = WORKS