As the title says, i got Ubuntu 13.04 Gnome Remix edition, i installed some Gnome Shell Extensions (Application menù, Netspeed, Impatience and User Themes) but i found out that they are not the cause of this problem. Anyway, i log in, but when i "change user" (it doesn't mean that i log in to another user, i just go back to the GDM), and then i log back to my session, the shell crashes and i don't see any upper bar, and the theme is now ugly. If i'm lucky, i'm able to start a terminal, run "gnome-shell -r" and restart the shell, but then i have to set again my extensions (which are now disabled) and change again the shell theme. I remember something about gnome-session-daemon, but all i found on the net did nothing to me.
Thanks in advance!