Wmbusmeter add-on does not connect to mqtt server

The wmbusmeter add-on does not connect to the MQTT Server. The logs show following message:

[12:19:04] INFO: Generating MQTT configuration ... 
[12:19:05] INFO: Broker will be used.
[12:19:05] ERROR: Got unexpected response from the API: Service not enabled
[12:19:06] INFO: MQTT Discovery cleanup...

In the logs of the MQTT broker, I can see a successful connect and a disconnect immediately afterwards:

2024-04-21 12:19:06: New connection from on port 1883.
2024-04-21 12:19:06: New client connected from as auto-73E3C20F-3701-706C-1620-70AB707A4F22 (p2, c1, k60, u'wmbus').
2024-04-21 12:19:07: Client auto-73E3C20F-3701-706C-1620-70AB707A4F22 disconnected.

The wbusmeter add-on is obviously expecting something special. Can somebody give me a hint?