Similar thing happened to me, and it seems to be a problem in the ribbon cable connecting to the screen. You must've done something in the process of installing the backlight that has damaged the ribbon cable, therefore the lines have appeared.

When I asked the same thing nobody came with any solution, and neither I found any when I looked over the web, I'm afraid.

that's what i ended up googling up, but i'm not totally convinced it's solely a problem with the horizontal ribbon cable.  that problem results in only one line dying, and the position of that line corresponds with the location of the particular break in the horizontal cable.

in this case, new lines end up dying over time, and there doesn't seem to be any pattern to which line will die next.  i suppose from the picture the dead lines are mostly in the upper half of the screen, but i'm keeping my eyes open for a counterexample.

i'm mainly interested to know if dead lines can be caused by an incorrect voltage rating on power LEDs (or any other installed LED's for that matter).

When I asked the same thing nobody came with any solution, and neither I found any when I looked over the web, I'm afraid.

the lines form one at a time.  they also form once every few minutes i guess.  the line in question would turn super black, and then fizzle out into a black line.

can anyone explain what's going on here?  is it definitely because of the new LED, or are there other causes?  i suspect it might be an improper voltage rating on the LED but i want to make sure before i try slapping it on another front PCB and possibly frying that one too.

(and yes i'm aware that horizontal lines are mostly untreatable, although i'm hopeful that this horizontal line behaviour doesn't completely match with what i've been able to google regarding horizontal lines)