MacOS – What geographic location is shown in the macOS High Sierra default desktop image

That is a nice way of starting a random screen saver. If, however, you'd simply like to start the active screen saver, you can use this in Mountain Lion:

tell application "System Events" 
    start current screen saver
end tell

Had the same problem and it is a simple fix.

1. restart your computer
2. when it "chimes" hold down "command" and "R" until a menu pops up.
3. Go to "Utilities" at the top of the page menu bar
4. Go to "terminal"
5. In the terminal type "resetpassword"
6. Follow the instructions. You may need you Apple ID. My Administrator account does not have an Apple ID, so I just reset the Administrator password without needing an Apple ID.

If you are not able to sign in with other accounts on your computer, go to your administrator account and make sure to uncheck Enable parental controls for those accounts in under System Preferences - Users and Groups. Then the other accounts should work fine.