(GOEDENAVOND)
The boxes with 12 leds need to be modified by putting a resistance of 82ohm in line with relais K3, the boxes with 4 leds are already modified with this resistance.
I remember you had a box with 4 leds, so i think your problem should be the cable ore the little print that is between the box and the display.
If you unplug and plugin the batterycable to quick you also get a dark screen, just unplug and holdon for a few moments before plugin again.