Interesting. I suspected it was some kind of short circuit, so I decided to screw off the mainboard, just to check whether any solder points on the back of it were connected or anything like that. As I couldn't find anything, I screwed it back in place and reassembled everything, after which the printer worked as it should. Perhaps some small piece of wire that previously caused the short circuit fell off or so… Regard this as solved, unless you have anything to add.