MQTT continually dropping connection

Continuing the discussion from MQTT gone on EmonBase:

It seems to me there are now quite a number of threads reporting this issue, with exactly this set of log entries. It seems the Mosquitto service is continually dropping the connection made by the phpmqtt_input script.

It would seem to me, that something outside of Emoncms may have changed.

I am wondering if Mosquitto has been updated and is handling QOS 2 differently. I found a couple of items that suggested the Mosquitto service drops a new connection when it thinks it is the same as an existing connection.

Just following on from a comment by @pb66 in the linked thread, could the issue be partly that the phpmqtt_input script is trying to subscribe to a non-existent topic as the emonhub has disconnected?

Unfortunately the onSubscribe callback does not return any status information.