Hi Elmar,
sorry that nobody has answered to you before - it may
be an oddball problem that no-one else has had or your post may have
just slipped past anybody who might know.
Anyway, to me it sounds
like the frame of the cloth is somehow loose - the bass drivers might
have problems, but it would be unlikely to be cured by you pressing on
the frame.
The full service manual is, of course, on site for
silver and gold members, but I'll attach a pic of the fret removal. I'm
not near my Beolabs so I could try it and I don't quite understand the
picture either, but perhaps you do, and manage to fiddle it back to
correct shape during the holidays