I have the same problem, but I do not know what action causes the blockage.
The only way I found and killed gnome-panel.
Why I do ctrl + alt + F1, then once connected:
sudo killall gnome-panel
Fresh install of 12.04 with gnome-session-callback / gnome-panel - no effects
Gnome Force Quit applet on gnome-panel.
I have the same problem, but I do not know what action causes the blockage.
The only way I found and killed gnome-panel.
Why I do ctrl + alt + F1, then once connected:
sudo killall gnome-panel
Fresh install of 12.04 with gnome-session- callback / gnome-panel - no effects
Gnome Force Quit applet on gnome-panel.