Hi!
Thanks for your reply. I also did notice that the voltage divider behaves a bit 'weird' (lower voltages, quite a bit) when there is load on it. Fortunately, today my voltage regulators came in. So I just gave it a try (with 4x AA): Nothing
Hooked up the RPI again as a test, and it worked.
So, then I decided it might be wise toconnect the grounds of the GND out of the regulator and the GND of the Arduino Pro Mini.. and it finally started to give a proper status signal! So That was very likely the reason. I am just not always so comfortable connecting different grounds together, because it seems rather "odd" to me. The path of an electron is sometimes a mystery for me 