Ubuntu Switch User

The only outstanding issue remaining with Ubuntu (assuming the next system update properly updates the Grub configuration file) is that switching users doesn’t work. If I lock the screen in my session, I can choose switch user and log in as someone else. But when I log out of the second session, the screen is blank except for the mouse cursor. Ubuntu can be restored with Ctrl-Alt-Backspace (the X11 restart sequence), but my first session is lost. More research is required.
Update: If I select switch user, then cancel, I get the same blank screen. However, if I suspend (Fn-F4), the session will be there on resume.
Update: This morning my wife did a switch user from my account. When she logged out, the blank screen was there. I suspended with Fn-F4, resumed, then everything was fine. …Except now the middle mouse button scrolling doesn’t work.