if you're running 144Hz that might be causing it, i have this monitor and it plays up at 144Hz, screen flickers and such. im guessing you've already tried putting it to 120Hz?

There's nothing wrong when I look at the pic you linked, it's all just dark grey.
I see flickering when I lower brightness on mine (70% or lower), but that's still only noticable on certain backgrounds. Have you tried setting the brightness to 100%?

I just googled some ancient forum thread. Is there anything close to your monitor that could cause unwanted interference or magnetism (like a cellphone)? I'm not sure if this applies to modern LCD panels though.

Also, are you still using your old monitor? I've had some dual monitor setup issues caused by AMD drivers.

Slightly off topic, but let me guess, you have a 120+hz and a 60hz monitor and with lightboost the refresh rate of the 120hz monitor drops significantly? I have exactly that problem running two monitors with an AMD graphics card that seems to be caused by lightboost. It's so annoying I've taken to not using lightboost. Have you found any fixes if that is your problem?

Why is everyone so obsessed with using lightboost lately?

On topic: the image just shows as a big grey box for me, but in the top right of my Asus VG248QE (same panel as the XL2411T) I notice some grey lines which I assume is what you mean. I haven't ran lightboost for a while due to the issue I mentioned in post 4 but if I remember rightly, the scan lines were more visible with lightboost on. Unfortunately, I can't offer a solution other than don't use lightboost.

Because it reduces pretty much all motion blur in the game. Let me put it this way, 120hz is noticeably better than 60hz, but 120hz with lightboost is still noticeably better than 120hz. It's something you really have to see to understand, but imagine no blurring at all on your monitor.

because it's really good. no motion blur whatsoever

Okey, setting it back to 120hz fixed that.. didn't work before because lightboost was being a poopy head.. Now it's normal 120hz with no scanlines. Will try to add it again, maybe it was acting up because it changed from 144hz to 120hz lightboost rather then 120 to 120hz lightboost?

Yea no difference with lightboost :/

try running it in strobed 120hz. I find it much easier on the eyes after playing for a while and it supposedly results in even lower motion blur.

Dunno how you set it up, but this is what I use.
http://www.blurbusters.com/zero-motion-blur/lightboost/

I used to use lightboost for ~2weeks I just can't deal with the bad colors and the monitor being basically black

No, not really. Refresh rate seems quite fine on my 6850 with a second monitor running at 70Hz. What I'm getting are occasional full screen artifacts (display shifting several centimeters horizontally, massive flickering, etc.). I recently found out it only happens when I overclock (Overdrive) the GFX card.

Ah, fair enough then. Haven't found anyone else with my issue yet then :(

I get the same issue on my XL2420T. Scanlines present in the top right corner of the screen when Lightboost is enabled, most easily noticeable in the killfeed.