It finally works!!!
John, that was the solution - the 1k Resistor in the TX line of the loop controller circuit.
I was a bit afraid of bridging it first , soI changed it to a much smaller value - and finally the voltage dropped low enough so
that the Raduino was able to recognize, decode and answer the incoming signal.
Thanks a lot for your help! I had not really been optimistic that I would be able to make it work. Your insight and advice really made the difference!
And thanks Gary also for your note. That had been one of my first steps, changing the the RX and TX lines.