The client identifier of the build in MQTT client looks like this: "Zabbix agent 2 5.4.12 ab-12-cd-34-ef".
This does not meet the MQTT specification (see MQTT Version 5.0). It says:
The Server MUST allow ClientID's which are between 1 and 23 UTF-8 encoded bytes in length, and that contain only the characters "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".
If I try to connect to CCU-Jack (mdzio/ccu-jack), the connection is refused with the error "identifier rejected".