Any update on this issue? I have a similar behavior with my openhab 3.4.4 (MQTT thing stops working) and I found out that it happens if the MQTT broker sends invalid messages, e.g., a “null” message, for whatever reason. Since other MQTT clients, such as MQTT-Explorer or mosquitto_sub continue to work, I assume it is aproblem of the openhab mqtt plugin and I opened a bug report, see [mqtt] MQTT thing stops working after receiving "null" message · Issue #15282 · openhab/openhab-addons · GitHub
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