*** This bug is a duplicate of bug 123078 ***
https://bugs.launchpad.net/bugs/123078
YES, your right : the gnome-power-manager was not running
(Système/Préférences/Sessions:Gestionnaire d'Energie).
Activating it (the gnome-power-manager) solved the bug.
It really seems that this bug is a dup
*** This bug is a duplicate of bug 123078 ***
https://bugs.launchpad.net/bugs/123078
** This bug has been marked a duplicate of bug 123078
System -> Quit takes a long time to appear
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Xorg freezes 60 seconds on shutdown button activation
https://bugs.launchpad.net/bugs/255046
You receive