Yes I searched for MQTT and i also found that thread, but unfortunately skipped the important part of that thread

This is the answer that I searched for.

Thank you

Is there only one client allowed to be connected to the MQTT-Gateway at the same time?

When I connect a MQTT-client on my PC to subscribe to the MQTT-Gateway everything works fine and I get messages.
Now I power on my Rasperry with OpenHAB --> OpenHAB can't connect to Gateway.

The other way round it is the same.
Power on OpenHAB --> connected with gateway
MQTT-Client on PC --> Connection not possible.
Power off OpenHAB --> MQTT-Client on PC can connect.

Is there a configuration to allow multiple connections or am i doing something wrong?

I've just found this thread, which describes the Convertion from 1.3 to 1.4:

http://forum.mysensors.org/topic/172/convert-sketch-from-1-3-to-1-4

I think, the following would do the trick, i will try it, when i am home this evenening:

// process incoming messages (like config from server) <-- insert line
gw.process(); <-- insert line
gw.sendVariable(MONITOR_STATION, CHILD_ID, V_TEMP, temperature, 1); change to --> gw.send(msg.setSensor(CHILD).set(temperature,1).setDestination(MONITOR_STATION)); // send float with one decimal point

Thank you for your quick reply Now it is clearer to me.

I do not understand how to set the Backend in the myconfig.h.

"you need to decide what signing backend to use. This is done in MyConfig.h"

But how? Just by commenting out the Pin Define?