Communication Broken with Mosquitto broker 6.0.1
Not sure if there is a bug listed for this as yet.
MQTT communication is broken with Mosquitto broker 6.0.1

From other issues ive seen it relates to using / in the clientname

I made posts in the Homeassistant Community Thread for this.
A similar issue described below

https://github.com/home-assistant/addons/issues/2020