Thanks for the quick response now we at least know what options there are for further testing
Although it prolly isn't the power supply, since we're directly getting power through a 9V Adapter with the Arduino on the Ext jumper setting.
It could be the noise however, we won't know for a few more hours because we have to wait for the academy to open so we can get an capacitator.
could it maybe have something to do with the length of the cables and/or the amount of connection points between the sensor and Arduino the cables are about 50 cm and there are 2 to 3 points of soldering in between? We already connected the sensors a while back without solderings and extra cable and then they were working perfectly.