thanks for the reply, Florian,
if you start it (eg gnome-keybinding-properties) from a terminal did you get any (usefull) output?
No; no output on cmdline, and no change in app appearance.
Did you rememver something you did one or two weeks ago (that we could blame)
I regularly "apt-get upgrade", but I couldn't say exactly when I noticed it, unfortunately, nor exactly what pkgs were involved :(