I just realised I am using a mkIII cable, which is not fully wired. Could this be a problem?

No, it was a fully wired cable. Again, no luck

Clutching at straws now - have you checked the MCL plugs to you main system are in the correct sockets? I doubt it would work at all if not though!

I checked it all! It seems to me like it is some sort of programming or software issue.

When I turn on the system and I press SOUND or STATUS, the volume drops at once, as if the MCL 2 AV doesn't amplify anymore. Then I can turn up the sound to normal level.

Maybe i'm way off here, but are the batteries of the terminal okay ?
Reason i ask is that perhaps the terminal (with low battery power) sends a code similar to "RESET"
when you press SOUND or STATUS.

Just guessing....

On all my terminals, a beolink 1000, beo 4 and the master control panel it's the same behaviour.

Is there no B&O technician on this forum, or perhaps someone that recognises this? I can't be the only one that has ever encountered this, I can imagine.

I have a similar system myself and several people that i know have a similar system.
So far i haven´t encountered the exact same problem, there have been issues with too low volume,
but the reason for that was a too low volume in main room.