Sorry I am just an idiot.Sorry I am just an idiot. I wanted to measure the voltage on the keyboard. Then I noticed something: I have soldered the LED array to a very big pinbig pin, that I thought it would be the ground (from my experience the ground is mostly found on that big pins. But it was D+, a data wire. I have then resoldered this wire to the real, very small −ground. Now it works. The LEDs stay lit.