Well, I'm back again, because it broke again ... and I fixed it somehow again.
I was playing around with my sensor to get it up and running the way I want it to and resetted its node id.
It couldn't get a new node ID and I was back where I was yesterday.
Deleted everything, rebuild the complete flow, restarted everything. Nothing helped. So I added debug messages everywhere and also had a look into the raw Serial protocol here: https://www.mysensors.org/download/serial_api_20
It looks like you don't need the encode and decode, at least for me the encode after myscontroller totally destroyed the payload. Why did it work yesterday? I have no idea.
The payload looked fine now without the decode and encode according to the protocol, but it still didn't work. I noticed that the incoming TCP payload always ended with a linefeed "\n", so I added a custom function between the myscontroller and tcp output to also add a linefeed to the outgoing payload and it's working again.