@crankycoder in 2.2 with appropriate binding mysensors is working pretty well. All variables are presented with mysensors lib 2.2.
I only updated openhab to 2.3 as this release was presented on my debian system. But I didn't get 2.3 working with mysensors mqtt.

Another thing is that sensor and gateway need a minimum distance. I had gateway and sensor on my desktop for testing and had problems. So I put away the gateway 2 meters and problems are gone. Just a try.

@timo I found the fault in my configuration: it was the missing secure=false config flag in the org.eclipse.smarthome.mqttbroker.cfg config file. I have no idea what happens when the flag is missing but without it one doesn't achieve a valid connection.

name=mosquittomys
host=192.168.96.8
port=1883
secure=false  <-- important !

So now I have a working 2.3 installation! Thanks alot for your support.

@bns unfortunately not really. What I learned from reading alot on different internet pages is that there are many proprietary implementations of the standard IEC 62056-21 protocol.
The Landis & Gyr UH550 expects 40 NUL chars before the initial sequence "/?!<CR><LF>". This seems to be a kind of wake up of the IR interface and has to be sent with 7 bit and 300 baud. After that the meter answers with its model number and than switches to 2400 baud with 7 bits and parity and then sends the rest of its current values. But unfortunately not common used OBIS datagrams but only the correct form with own OBIS number format.
Hope this is of any help.

@diogoc do you have a working sketch? I worked on the same topic for a landys & gyr uh50 heat counter. It's not a real 62056-21 protocol, initialization is different.
Best regards

@masmat I just wanted to understand your configuration. So I read through whole thread and saw your problems began with upgrade to OH2.3.
I mentioned eventbus as you wrote something about eventbus.cfg.
You need eventbus only if you want to send all oh events to mqtt broker.
I wanted to see your mqtt bridge configuration. You can either configure dynamicly via paper ui or you set it up via things file. The later is what I did. So here is my things file with bridge configuration.

Bridge mysensors:bridge-mqtt:gateway [  brokerName="mosquittomys",
                                        topicPublish="mysensors1/in",
                                        topicSubscribe="mysensors1/out",
                                        startupCheckEnabled=true ] {

}

Here the brokername has to be the same as in mqttbroker.cfg. Fix topic names to match your mysensors gateway.
Hope this is of any help.

@masmat two things come to my mind: I have two different broker-names for eventbus connection and mysensors connection. For eventbus binding you need classic mqtt1 binding with its own configuration like this:

mosquitto.url=tcp://127.0.0.1:1883
mosquitto.retain=false
mosquitto.clientId=openhab
mosquitto.user=user
mosquitto.pwd=pwd

And second: can you show your mysensors bridge configuration? Not shure but debug log looks like an ethernet bridge and not a mqtt bridge.

@timo I found the fault in my configuration: it was the missing secure=false config flag in the org.eclipse.smarthome.mqttbroker.cfg config file. I have no idea what happens when the flag is missing but without it one doesn't achieve a valid connection.

name=mosquittomys
host=192.168.96.8
port=1883
secure=false  <-- important !

So now I have a working 2.3 installation! Thanks alot for your support.

@timo with 2.2 I had two configurations which were very similar.
I have one mosquitto mqtt broker on localhost, version 1.5.
The mqtt1 binding is for my mqtt-eventbus connection is called mosquitto and the config file (mqtt.cfg) is as follows:

mosquitto.url=tcp://192.168.96.8:1883
mosquitto.retain=false
mosquitto.clientId=openhab
mosquitto.user=user
mosquitto.pwd=pwd

With OH2.2 this configuration works. It also works with 2.3 as I can see all openhab events on my mosquitto.
My mysonsors binding is configured by org.eclipse.smarthome.mqttbroker.cfg config file which looks as follows:

name=mosquittomys
host=192.168.96.8
port=1883
retain=false
username=username
password=password
clientId=openhabMys

I also tried without authentication and also without clientId, all to no succuess.
I cleaned /var/lib/openhab2/cache/ folder and /var/lib/openhab2/tmp/ folder.
I looked at /var/lib/openhab2/config folder for resulting runtime configuration, but all looks fine. Do you have any further idea?

@timo unfortunately this also does not work for me. I followed your manual very well but I always get errors. It starts after installing feature:install esh-io-transport-mqtt and mysensors binding:

16:59:57.772 [ERROR] [protocol.mqtt.MySensorsMqttConnection] - MQTT connection offline - Reason unknown
16:59:57.806 [INFO ] [o.transport.mqtt.MqttBrokerConnection] - Starting MQTT broker connection to '192.168.96.8' with clientid paho1135997459230242 and file store '/var/lib/openhab2/mqtt/192.168.96.8'
16:59:57.867 [ERROR] [protocol.mqtt.MySensorsMqttConnection] - MQTT connection offline - {}
org.eclipse.paho.client.mqttv3.MqttException: MqttException
        at org.eclipse.paho.client.mqttv3.internal.ExceptionHelper.createMqttException(ExceptionHelper.java:38) [229:org.eclipse.paho.client.mqttv3:1.0.2]
        at org.eclipse.paho.client.mqttv3.internal.ClientComms$ConnectBG.run(ClientComms.java:604) [229:org.eclipse.paho.client.mqttv3:1.0.2]
        at java.lang.Thread.run(Thread.java:748) [?:?]
Caused by: java.net.SocketException: Connection reset
        at java.net.SocketInputStream.read(SocketInputStream.java:210) ~[?:?]
        at java.net.SocketInputStream.read(SocketInputStream.java:141) ~[?:?]
        at sun.security.ssl.InputRecord.readFully(InputRecord.java:465) ~[?:?]
        at sun.security.ssl.InputRecord.read(InputRecord.java:503) ~[?:?]
        at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:983) ~[?:?]
        at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1385) ~[?:?]
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1413) ~[?:?]
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1397) ~[?:?]
        at org.eclipse.paho.client.mqttv3.internal.SSLNetworkModule.start(SSLNetworkModule.java:89) ~[?:?]
        at org.eclipse.paho.client.mqttv3.internal.ClientComms$ConnectBG.run(ClientComms.java:590) ~[?:?]
        ... 1 more

and reconection starts every 10 seconds.
It must be interference with mqtt1 bindung that I use for eventbus.
The mqtt-bridge remains Offline.
What is wrong?

@TimO : are there any news concerning compatibility to version 2.3?
Thanks for your work!

@madelle-kamois this issue is already discussed in this thread. In 2.3 mqtt Implementation has basically changed. Develloper @TimO is working on it.

@crankycoder in 2.2 with appropriate binding mysensors is working pretty well. All variables are presented with mysensors lib 2.2.
I only updated openhab to 2.3 as this release was presented on my debian system. But I didn't get 2.3 working with mysensors mqtt.