After installing VirtualBox on my laptop, I locked my computer and went to lunch. When I returned and woke my laptop, I was greeted by an "Authentication failed" error and a button that read "Switch to greeter". Clicking the button allowed me to login as normal. I thought this odd, but did not think anything of it.
However, the same thing happened when returned from a meeting and found the laptop locked. It happened a third time after a trip to the coffee machine. I knew it was time to dig into it and figure out what went wrong.
Some quick Googling did not find a lot of fixes. I did, however, come across this post on Medium which did fix the issue.
From the terminal, I ran the following commands:
$ sudo apt install --reinstall lightdm $ sudo service lightdm restart
NOTE: Your application will close after running the second command. Make sure they are closed before running it!