I have found how to manually assign the shortcut. It is hutterly broken from a UI design point of view, but works otherwise:
1) go to gnome-control-center / keyboard / shortcuts
2) Launch a terminal says: "Unactivated" (or something like that, here it's "Désactivé")
3) don't try to click on +-, click on the word "Unactivated".
4) turns into "New shortcut": press ctrl-alt-T
Just found this out of luck, but the UI should really be fixed.