So, I don't know, what I have done, but now it seems to work.
Probably it was the following: Depending on the scenarios, I need from 1 to 4 PIR sensors. Now I changed the Code, so that only the connected PIR PINs will be avtivated with "pinMode(PIN_PIRi , INPUT);"
Perhaps this was the point, but I'm really not sure.
Now I'm working on the power consumption, to high, 17 mA.