Ubuntu — switch to greeter error

Recently I’ve started getting an error on the login page of my ubuntu laptops (16.04 and 18.04), both running Unity.

Whenever the laptop is put to sleep or the screen turns off it wakes up with the error saying:

Authentication Failure Switch to greeter

There is an easy fix for it that has worked for both Ubuntu versions:

sudo apt-get install --reinstall lightdm

Hope this helps :)