@electrik: I found the solution. It was my misunderstanding of your advice. As you mentioned to add the \n then I thought it is the \n in the configuration of the Serial-Ouput Node, see here:

In the Output section you can "Add Character to output message". Apparently this is not equivalent to adding \n into the
payload itself. Now my paload looks like this:

and the Node is receiving it, great :-)
Thank you very much for your help @electrik

Thanks for your feedback frits.
This is what I found searching this forum and already replied to my own post, but it takes some time the post to become visible.

With best regards
Alberto

Currently I implemented some nodes sending data to a Serial-Gatway connected to a RasPi.
I'm running Node-Red on this RasPi and and the data area stoet into InfluxDB. Everything is working fine.

So now I'm trying to send some data from Node-Red to an example node.
First I verified that the node is working fine, when sending data to the Gateway.

For sending the data I'm using following flow:

It sends the number 275 to my Node_ID 29. The type is V_VAR1.
The Node-Red Debug shows, what is sent:

The loop in the node is a state machine, where I added a wait() just to slow it down while testing.
The cases are empty functions so nothing happens there.

void loop() 
{
   Serial.println("Runinng Main Loop");
   wait(3000); 

  // Main-Lopp for state-machine
  switch (Current_State)
  {
     case CHECK_LEVEL:        Check_Water_Level(); break;  
     case FILL_WATER:         Check_Water_Level(); break; //same function for both cases. 
     case PUMP_ON:            Pumping(); break;   
     case SUSPEND:            Suspend(); break; 
     case REMOTE_CONTROL:     Remote_Control(); break;
     case SLEEP_MODE:         Start_Sleep_Mode(); break;
  } 
}

The receive function includes only some print commands to show me when the data was received:

void receive(const MyMessage &message)
{

  Serial.println( "Message received:  ");
  Serial.println( message.type);
  Serial.println( message.getUInt());
}

Unfortunately nothing happens, when I'm injecting the data in Node-Red.
I scanned the forum but didn't find the right postings helping me to understand, what my error is.

Thanks in advance for any advice.

@frits: if the MQTT GW do not support the dynamic ID assignement, then what happened to my other sensors (those based on the Nano) :thinking_face: They got the Node_IDs from the GW. All the sensors are on the same GW.

@frits : This is my first GW, but during my early experiments I run it as an Ethernet GW, then I reconfigured to MQTT.
Currently I don't have a controller running but I'm using Node-Red + Influx-DB + Grafana.

So probably during these first experiments, while in Ethernet mode, the Node_IDs were distributed.
I will erase the EEPROMs by the next opportunity and will check the behaviour of my Nano sensors after re-boot.

Thanks for your explanation frits, it helped me to learn more about MySensors.:+1: