Sorry for replying in English.  It is most probably a problem with the keyboard's ribbon connector which also forms the contact pad behind the buttons.  It will feel like it is working, but the contact pad needs replacing because some of the traces are broken.

I repaired my BC2300 (which uses the same part) by buying a repair kit from B&O as follows:

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Danke für die Antwort und Erklärung! Ich habe die Beosound mal aufgeschraubt um an die Tastatur zu kommen. Muss ich an das Teil auf dem Foto? Wenn ja, wie kriege ich das raus? Scheint sehr fest zu sitzen. Ich habe noch eine teildefekte Anlage (Tape, CD kaputt), kann die Tastatur von der einen Anlage in die andere getauscht werden?

Danke!

Thanks for the answer and explanation! I opened the Beosound to get the keyboard. Do I have to change the part in the photo? If yes, how can I get it out? Seems to be very tight. I have a second defect Ouverture (Tape and CD broken), can the keyboard be exchanged from one system to the second one?

Thank you very much!

I have attached a copy of the instructions that were included with the repair kit - they may be useful.  PCB5 is the display PCB that you have rotated backwards in your picture.   I assume that you disconnected the ribbon connector that you can see in your picture?

I can't remember how far I dismantled the keyboard, but I think I managed to get to the stage where it looked like this (picture from internet):

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The simplest solution would be to replace this complete piece from your other Ouverture., if that is in good condition.  (EDIT: I think the 4 light units will unclip to save re-soldering if you swap the panels over)

However, as it says in the attached instructions, the official repair kit requires your to cut off those 8 plastic buttons so that you can remove and replace the foil.  The repair kit then comes with 8 very small self-tapping screws to replace the plastic buttons and hold the keyboard together.

Many thanks again! Unfortunately, the keyboard from my seco nd Ouverture isn't in such good condition, which is why I'd rather buy the entire repair set. Any advice on where I can get it? Does it have to be ordered directly from B&O?

I bought mine from here but it was 12 years ago so I am not sure if it is still available: