This seems to be an issue with how Apple is serving updates, or the updates were interrupted while attempting to install --> The update(s) hang, or get not installed, but/or the system is trying to install a non existing update.
To solve the issue (possibly) is as follows:
1. Reset the PRam 3 times --> Hold the Option + Command + P + R keys down, until you hear the boot-chime 3 times in a row --> let go after the 3rd time and immediately hold down the Command + S keys. --> This will get you into the Single User Mode.
If you missed the Single User Mode --> Force Shut-down the Mac (hold down the Power key until black) --> Reboot holding the Command + S key down.
2. Once in Single User Mode, type the following commands:
/sbin/fsck -fy (there is a space after the k)
/sbin/mount -uw / (There is a space after the t and a space after the w)
After the above is completed, type:
rm -f /Library/Preferences/com.apple.loginwindow.plist (there is a space after the m, after the f)
rm -f /var/db/.AppleUpgrade (there is a space after the m, after the f and a dot before the A)
After that, type reboot
This should solve the issue.