Firewizard,

I figured it out :-D
For personal reasons I had to go back to the old situation...

The problem was within what you described, so I needed a MQTT out so to say.
And the second problem was:

This was my function:

switch (msg.topic) {
case "p1monitor/smartmeter/consumption_gas_m3":

msg ...

Firewizard,

I think you might be right!
I cannot test it anymore, since I decided to swap my Raspberry(1) for my Raspberry(2) (meaning now my
new bought raspberry(2) is directly attached to my smart meter P1).

In the above situation I got messages from RBPi-1, which was directly connected to the ...

I have been busy for two long days and I just can't seem to figure out why I can't get this to work :-(
I did (I think) exactly what was described in topic "Re: Get sensor values with MQTT into Domoticz"

Is there anyone who has an idea maybe?

Goal:
To get MQTT values from my second Raspberry Pi ...