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.