Another anomaly is that when I disconnect the power from the 5V of the arduino to the VCC of the shift registers the LEDs are still on, albeit much dimmer. I then measure ca. 30 mA in the cable going from a digital pin to the ST_CP pins. The LEDs are getting 3-4 mA which again doesn't add up. I don't understand why or how or where from exactly the LEDs are still getting their power.
Sorry if this is a stupid question, I am absolutely new to electronics. I am missing roughly 20 mA in my circuit. There should be 160 mA going in and out of my board considering that I connected 16 LEDs each drawing 10 mA(+-0,2mA). But I measure only 140 mA.