Hi all. I had the v4.1 horrors too. The solution for me was a thermo nuclear one. Had to start a new install from scratch. My best guess is that there are some sort of remnants/artifacts in an existing installation that make upgrading to v4.1 haphazard. I really think it has a lot to do with what your pre-4.1 setup was. Whether you had a manual MQTT setup in configuration.yaml, whether you had a MQTT setup via discovery, whether you had Home Assistant user, whether you had Local Users, etc.
This was my bad start point…
This was my way to happiness…
Clearly there are so many users complaining about one setup working for them, then not for others that try and follow the same steps. I believe my process follows the official docs/instructions accurately and delivers a stable working Mosquitto broker v4.1. Good luck!