Look in your syslogs (assuming that is where you configured mosquitto to log to) and see what mosquitto says is going on. The repeated connection lost implies that mosquitto is crashing then restarting.
Just hit the same issue (I believe)
My mosquitto log claims that OH is trying to connect AGAIN so it (mosquitto) drops the existing connection.
Any idea what to change ?
Make sure you use a different client id for each client that connects to the broker. When you reuse the client ID the broker things that the same client is trying to reconnect and closes down any connections it already has open with that client id. Note that the client id is not the same as username which is used for authentication. The default client id is “openhab” I believe for the mqtt binding.
Yes, it was caused by double ID because I had two OH instances running. One of these itchy-quirky-somethings you easily forget about in the heat of a migration. Thanks anyway.