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!