admin : finally it works. The key-binding SHIFT-W which I used before for the former PlaybackStart is kind of blocked. I set another key (SHIFT-J) and with that it works.
I think something was messed up yesterday when I set SHIFT-W for the toggle action and the system told me that it is already used for PlaybackStart (which in fact did not exist anymore and has been replaced by the toggle). I then deleted this double binding by clicking on the X. From then on the toggle did not work anymore with SHIFT-W. Thanks for your patience and help.
Very nice solution for Playback, especially the new option 'Go Back'.
