I've seen many people complain about this issue, and so I worked with Apple Care on this and they provided the fix below.
- Power off iMac (hold down the power button until it shuts off)
- Power back on
- When you hear the chime, press Command+S. This will take you to boot the iMac into a command line.
- Once at the command line, run the commands below.
Note: commands are case sensitive, so type them exactly as shown below. Also, you will need to hit Enter after each line.
mount -uw /
mv com.apple.loginwindow.plist oldlogin