One way to fix this is to install GNOME Shell and use that, as it's able to
switch users from the locked screen. You can wait for other suggestions, if
you prefer to not use GNOME Shell.
You can install GNOME Shell with:
sudo aptitude -R install gnome-shell
At first I had problems with logging in the GNOME session, so I installing
GDM. Later I found I had to use "System Defaults" session rather than "GNOME
session", otherwise it would only load a blank desktop and a lone mouse
cursor.
May you don't need GDM, but if you do, you can install it with:
sudo aptitude install gdm
