What's up with that screen corner?

As a matter of interest, try changing resolution and see if the effect changes position or goes away. If it doesn't change it might suggest an illumination issue or screen defect as you say.

If the monitor cable lacks those lumps that contain ferrite chokes try making a simple knot in the cable and see if that affects it (used to work with blurring on CRTs).

It's a weird one but seems not to be visible most of the time -- unless it bothers you or gets worse I'd be inclined to put up with it.

It's a defect alright, even without any kind of a video signal it's still there. I guess I'll RMA it and considering near future, avoid Acer and their cheapo monitors.