Comment 2 for bug 768824

All indications point towards 'Super+space' being grabbed by unity. In that case it can't be used.

libkeybinder will grab related keybindings like Shift+Super+space, and in this case the bug in libkeybinder is mostly that it should fail on this keybinding completely, instead of binding it partially.

For kupfer: wontfix: unity uses this shortcut, use something without Super?

See also Kupfer FAQ: https://live.gnome.org/Kupfer/FAQ#Can_I_use_Super.2B-Space_in_Unity.3F