Turned the monitor off and on again...not sure why that needs to be done but it works 😛

I have been wanting to reply earlier, but had to wait a couple of days to be able to post.

I have had this same issue with 2 of these monitors (the first also had an issue with the base light so was sent back), and since I haven't seen anyone else but you mention this I started to wonder if it was not an issue with the monitors, but my setup, unless I am just incredibly unlucky to get a rare problem with 2 monitors in a row.

I managed to grab hold of another graphics card, my card is a Zotac GTX780 and I borrowed an Asus GTX670, and had the same problem with the screen going dark with ULMB enabled. At the weekend I plan to take the monitor to hook up to a friends PC and see if it is perhaps a software issue on my PC.

When you say you fixed it by turning the monitor off and on again, did this permanently fix it for you? In my experience this only worked if the DisplayPort Deep Sleep function was enabled (which I prefer to have switched off), and then if you open and close the OSD the screen goes dark again, so it only seems like a temporary workaround.

I created a video showing the problem : https://youtu.be/-iOFk9HaPr8

Yep, I pressed the joystick to enable ULMB. You can see in the video where the ULMB checkbox is ticked and the screen dims as ULMB is enabled (which is confirmed by the text at the top right of the OSD). The monitor doesn't need to restart to enable ULMB, like it does to change the top refresh rate from 144 to 150+

Yeah but the monitor is already set to 120hz before I enable ULMB, the hz setting doesn't change from that. So there is nothing to confirm in that regard 🙂