VR Preview opens window but not in HDM

Hey guys,
The VR Preview option is available and I can launch it.
Even if I can see the VIVE controllers moving in the window that pops up on the screen, there is no image in the Headset of the VIVE nor does it track the Headset’s movements.

The windows that appears is totally out of size though and seems to be much bigger than the resolution specified 1280x720 in the advanced settings.
What could be causing this problem? I can launch my gym correctly with the usual commandline :
XXXXX mapname -game -vr -ResX=1280 -ResY=720 and it works perfectly fine on its own.
The Unreal Editor in VR mode is working perfectly with all the trackers
Any steam game works fine with the SteamVR and such.

The ONLY problem is with the VR Preview when in a gym in the editor.

What is wrong with this :frowning: