i had running a working OH2 (2.5) on an RPI3B with 1 MQTT client. ESP Easy as Temperature sensor.
Works fine. Other components are Alexa, Homematic and Sonoff and Habpanel
Last weekend I start to upgrade to OH3. And also from Jessie over stretch to buster.
Backup from SD-card and pray.
Now 99% is working again.Habpanel, Homematic and Alexa commands. All old data are still there.
The only bad think is the MQTT.
I have reinstall Mosquitto serveral times, found that its now in version 2, which needs security as default.
Actual the bridge told everthing is fine but the external device and the mqtt things write error bridge.
When I try to check with MQTT Explorer there is allways connected and disconnected and a red message displayed disonnected. I have use the OH config and install a password. I can see with winscp that there are now files with password and user in the Mosquitto folder.
Have someone the same problem, a solution or way where i can check to find my failure