Finally the device found 2 neighbouring nodes (in addition to the controller) and right now the device works well.
Occasionally the log shows significant delays, e.g.
RX REQ SendData 248 ACK RECEIVED from device in 4230ms
Anyway, the device works now!
@[5iver] @[mhilbush]: Thank you very much for your support 
Ah, about my Qubino Flush Dimmer ZMNHDD, which does not work with secure inclusion, I will open another thread on that problem.