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"
** Also affects: gnome-screensaver (Ubuntu)
Importance: Undecided
Status: New
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[nvidia] compiz displays white screen when locked
https://bugs.launchpad.net/bugs/160264
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