@nizoo91 Stick to addons.cfg in /etc/openhab2/conf/services and don't fiddle with the file you mentioned above.
With package=standard (look for the examples in the comments) the UIs should reappear.

@waspie I'm testing the binding against a mosquitto in my development environment (eclipse IDE) and it is working fine there. That makes it so hard for me to debug the issue.

@waspie Well, it's working for @Guillermo-Schimmel and it's working in my development environment. This bugs the hell out of me. But maybe that's a problem that will be fixed with a update to OH2.

@waspie Well, I do use an MySensors ethernet gateway with the MySensors binding which works fine.
In addition to that I have MQTT devices, for example may heating unit (ebusd) which I connect via Mosquitto and the generic MQTT binding to OpenHAB. MySensors is not involved here.
My advice: use the ethernet gateway with the MySensors binding and keep Mosquitto for the rest. There were some changes in the MQTT implementation of Openhab in 2.4 which lead to some problems, like the one in this thread and I've found some more, while trying to adapt the new configuration and I'm sure there will be some changes to that in the next release. I'm currently using two Openhab instances (2.3 and 2.4) in parallel till all problems are solved.

@waspie you're able to use more than one controller with the MQTT gateway. So while using Openhab for automation you may use My Controller for wireless node updates / firmware uploads. That is not yet possible with the binding.

@jimmy-loyens Could you please check if the embedded-mqtt-broker thing is online? I suppose not. If the embedded broker addon (misc) is installed OpenHAB automatically discovers this embedded broker, just add that thing from inbox.
Now connect your MySensors MQTT gateway (hardware) to the IP and Port of the embedded-broker. After that your MySensors-bridge configuration should work. The port configuration at this point is not needed, but should do no harm, it'll be ignored.

Documentation for the binding: https://github.com/tobof/openhab2-addons/wiki

I've still found no solution but if I use the new embedded broker in OpenHAB2 and connect the the MySensors MQTT gateway to that broker everything works fine.
But I'd no success in connecting the MySensors MQTT gateway to a working MQTT-broker (MQTT2) or in creating a SystemBroker (MQTT2).

So for those of you that are willing to use the embeeded broker, this works fine with the MySensors MQTT gateway.
Just use embedded-mqtt-broker as broker name in the MQTT gateway configuration.

@guillermo-schimmel OK, next question: how did you define the system broker named mosquitto? Because that's the one the MySensors binding seems to use. In a service (cfg) file? I've had no success defining one and maybe that's the key.

@guillermo-schimmel You've mixed up attributes from the MySensors binding and the MQTT binding in the bridge definition. brokerName, topicSubscribe and so on are attributes from the MySensors binding. But this won't do any harm.

How is the MySensors gateway defined?

Hey @jimmy-loyens !

I needed some time to investigate the problem and after looking into it, I've no clue why this is working for you @Guillermo-Schimmel .

With version 2.4.0 and the changes to the mqtt binding I'm not able to connect the MySensors binding to the MQTT broker connection configured in OpenHAB. The MySensors binding uses the MQTT implementation and connection of OpenHAB, it does not connect to the broker itself.

And now the funny part: my development system works fine, so I need to find a way to debug this problem.

@Guillermo-Schimmel Did you configure broker and things in PaperUI or in textual files?