I've booted the system just to send you the info about ck-list-sessions, sent it and then shutdown and this time it worked (for the first time). Today the system had a quite large update and also I have been using IceWM instead of Gnome these last days. If you want me to try to find what was updated recently, please tell me.
There's one thing, though, that may be useful to know: in order to enable 'reboot' and 'shutdown' from within IceWM I allowed /sbin/reboot and /sbin/halt to use sudo (without password) using the following line ALL ALL=NOPASSWD: /sbin/halt, /sbin/reboot
