Mosquitto add-on does not install because port 1883 already in use

I’m having some difficulties with Mosquitto the past few days. One device (Heatbooster) could not connect with mosquitto anymore:

2023-01-27 11:30:53: New connection from 192.168.1.134:50058 on port 1883.
2023-01-27 11:30:53: Client <unknown> disconnected, not authorised.

Weird, as credentials were ok and not changed, before that it worked fine for weeks. Rebooting everything…

  • Mosquitto add-on
  • Mosquitto integration
  • HA
  • Heatbooster firmware update
    …didn’t resolve anything.

Then I decided to remove the integration and add-on. Re-installing the add-on got me the message that port 1883 is already in use. Which is weird, because I don’t have another instance running afaik.

  • There’s no mention of mqtt in configuration.yaml
  • netstat -an doesn’t show anything using port 1883

Current HA: HA was al helemaal bij (Home Assistant 2023.1.7, Supervisor 2023.01.1, Operating System 9.4, Frontend-versie: 20230110.0 - latest)

I’m lost, help apreciated!