I have found the problem:
In RUN mode, the block "Set Communucation Channel" is not executed. The units work on the default channel 6. I had selected channel 1, as it is emptier for me. After downloading into the device, however, the channel block was executed and the device ran on channel 1. That is why the devices did not find each other. After I changed the script to channel 6, it works as it should.
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