After several tries I dont understand how this thing works!

See diagram attached. E is always the ground.

On AUX port, pins C and D are the inputs. I get full level record from here.
With the same cable on AMP port I have no signal at all.

On AMP port, pins A and B are the inputs. I get full level record from here meaning C and D are the outputs. 
When I connect it with the same cable I use for my Beocord 1200, I get full playback but record from one channel only.

So I don't get where the second channel is gone.

Tried with different cables.
Tried to plug a Beocord 5000 to the AUX -> Work as expected.
Tried to plug a Beocord 5000 to the AMP -> Nothing.

Am I missing something?

The AUX and AMP sockets are not the same;

AMP is used for connecting to amplifier/receiver, for playback and recording. This is the socket you will normally use.

AUX is used to connect two tapedecks directly (tape copy), so only has inputs for recording (playback pins are grounded to prevent loops).
Further, the pins are mirrored (!) to allow using the same cable as you would for the amp.

Always keep in mind, that modifications could've been made by previous owners.
Perhaps the socket in your deck was rewired - or the one in your Beocord 1200.

It's also possible that a socket has bad contact (or even an internal break).
With a 1mm flatbladed screwdriver you can prod the socket contact pins a little to add more contact tension. 

Martin 

I'll try another cable then.

Thank you.

Or maybe then it could be the beomaster which is defective?

It was a wrong cable.

Apologies.