PC refuses to boot with Mini DisplayPort plugged into GFX-card

[Rippin_Kittin]

Setup:
ASUS ROG SWIFT ---> DisplayPort to Mini DisplayPort ---> ASUS R9 295x2 / PC


So I was going to start up my computer for the first time but realize the computer shuts down after one second
after pressing the start-button. The monitor is also showing the sweet message of "no signal".

This is what I have tried out...
#1: As soon as I plug out the DisplayPort to Mini DisplayPort-cable between monitor and graphic card, the PC
sucessfully boots up.
#2: I have taken the graphic card out and used the motherboard's integrated graphics along with the DisplayPort
on I/O port instead and also with the standard DisplayPort-cable that came WITH the ASUS-monitor. Then the
monitor wakes up and PC starts without any problems. I also noticed in BIOS that my Devil's Canyon was up and
workin after the BIOS flash, so no problems with that either.

My theory is that either the cable (DisplayPort to Mini DisplayPort) I'm using from monitor to graphics card is
faulty, or there must be some issues with the Mini DisplayPorts on the graphic cards itself.

The cable which came with the ROG SWIFT monitor is a standard DisplayPort which I used for the I/O port.
Unfortunately, the R9 295 x 2 has only Mini DisplayPorts, so I can not try that cable on it.

Any idea what to do?
Would a more expensive/qualitative DisplayPort to Mini DisplayPort-cable do the job right perhaps?
How common is it that there are "bad" DisplayPort-cables on the market?

I have to admit, the DisplayPort to Mini DisplayPort-cable I bought was cheap.

When a PCIeX16 graphics card is installed in the first PCI slot the onboard VGA and Display ports will be
disabled automatically, right? That setting is set on "auto" in the BIOS by standard.