Default [Q] NSTwGlowlight, half IR sensors acts wrong

Hi,
I have recently bough a Nook Simple Touch with Glowlight (BNRV350). Used, no guarantee. It looks fine (has only a minor scratch on the light dispersing layer). Problem is - the lower half of the touch sensors give misplaced readings. For example - during the registration process I cannot get through the language selection screen. I can correctly mark English, but when I want to press an arrow to go further, Nook changes my mark to French, instead of getting me to the next screen. Cleaning borders of the screen with microfiber cloth and cotton swabs did not help.

I have another NSTG working fine, so:
1. I tried to copy its stock system backup and loading it to malfunctioning one. It didn't help. I could not even unlock the screen (although I could press "power off" or "cancel" buttons after holding power button).
2. I tried swapping mainboards (IR sensors are on them, not in the screen assembly). And It turned out, that wherever mainboard of the faulty unit is, the touch does not work properly. I tried to blow out the dust from those misbehaving sensors hoping that it just somehow got on the motherboard, but it did not help either.

So the question is: What else can I do to make those IR sensors work properly?

Thanks in advance for looking into the problem!
(Sorry for my English, I'm used to reading, not to expressing my thoughts with it.)