Quoted from tsool:
Now I need to understand how the switches work in order to decide if something is broken or not. Does it work like a sensor barrier that the beam is cut when the switch is "on"?
Exactly right - most likely the IR emitter for that switch is not working. Easy to test using your cellphone or digital camera. It could be a broken trace, resistor, or bad emitter. Your multimeter will show you the way. Those trough boards have a hard life due to the heat and constant vibration!