Just received the KIT1 and connected on-line. All works as far as I can tell except for the hardly readable display. Noticed the PCB on the upper end/edge is bowed. If I place pressure on the upper corners of the display it seems to correct itself with perfect dark contrast. I thought that fixed the issue, yet about an hour later the display dimmed again. It appears to be an internal contact issue between the display and underlying PCB. Checked the mainboard PCB header pins, solder joints, etc and nothing stood-out.
It seems the zebra strips (conducting rubber) that connect the display elements to the display’s PCB do not make proper contact due to the bending of the PCB.
You can try to even out the PCB bending by heating up the connector solder joints. You have nothing to loose there.
If all fails, there are lots of Nokia 5110 display sellers out there, just get a new display module…they are cheap too.