Oh man thanks a lot. That probably explains whyI get correct readings in some cases and wrong readings in other.
 
The pins are connectet to the IO Pins of the display. So I assumed I don't need any additional reistor, because they will be driven by the other device. I might be completely wrong though.
 
So a first fix would be, to set them all high before reading, right?
 
Would it be reversed if I would have set them to Level::High initially? Ie. I would need to set them Low?