MQTT - Client disconnect

Hello - I'm troubleshooting Client connection issues (in a wider context of controlling a Shelly Plus Plug S).

To run my tests, I use an iOS app called MQTTool.

https://apps.apple.com/us/app/mqttool/id1085976398

(I also tested another app to do tests, with same results)

My Comfortclick MQTT setup :

I have one client configured ("client2"). And it's enabled.

1) If I use MQTTool, and use "client1" as client id, I'm able to connect and send a message to it:

Monitor :

2) If I use "client2" as client ID (that is configured and enabled in Comfortclick), I'm not able to connect (there is a tentative that is immediatly disconnected). Moreover, when I connect, "client2" gets removed/disabled in Comfortclient Clients list.

Monitor log :

After the disconnect, the client2 reappears in the Client list :

Debug is showing :

Any suggestions ?

Thank you