Hi, I have a similar problem with an LG 27UD69-W from April 2017.
It gives a sort of 'square inside a square' ghost image that is really, really green.
Resets etc don't seem to help. Interestingly, it DOES seem to work without problems when it is significantly colder and critically less humid.
I suspect hardware somehow, maybe power leaking across a circuit it shouldn't through electromagnetic field, that is affected by humidity? (higher temp means more moisture can be held in the air, exponentially so).
It starts at around 22 degrees Celsius. If it's colder than that, the screen runs fine.
Since it's outside of warranty I expect LG won't help. Maybe something on the monitor's circuit board needs to be electromagnetically insulated.
But I'm not really a hardware guy, so these are just guesses. But I figured I'd post it here, you could try running it with an airco pointed at it or in a colder location and see if it helps.