Are you using the mosquito mqtt addon? Did you recently upgrade to v4.1 of the addon?
If so you need to add an ACL. Mosquito update 4.1 - ACL messages in logs
I still find v4.1 a lot less stable than 4.0 was. I’m getting loss of contact between the broker and HA every few days of continuous up time.
The solution to loss of communication ( after adding an ACL) is just to restart HA. So it appears to be a problem with HA talking to the new version of the addon.