It seems to be gnome-screensaver / nvidia driver's fault
Quick fix, do the following on both accounts you're using fast user switch:
1) Disable gnome-screensaver: System > Preferences > Screensaver
2) Right-click on "Fast User Switch" applet on the gnome panel bar,
choose "preferences", uncheck "Lock screen after switching users"
It seems to be gnome-screensaver / nvidia driver's fault
Quick fix, do the following on both accounts you're using fast user switch:
1) Disable gnome-screensaver: System > Preferences > Screensaver
2) Right-click on "Fast User Switch" applet on the gnome panel bar,
choose "preferences", uncheck "Lock screen after switching users"