Problem

After locking screen on Ubuntu, the next time you login, you see the message "Authentication error" on login screen and cannot type your password anymore to login.

Solution

On the login screen, Press Ctrl + Alt + F3 to open a terminal, or use SSH to access from another computer. To unlock all sessions, type sudo loginctl unlock-sessions. Then press Ctrl + Alt + F2, for example, to go back to your (now unlocked) session.

This does not solve the underlying problem, but lets you quickly get back to your session to save your work and try configuration changes, without having to reboot.

References

• https://askubuntu.com/questions/1032624/ubuntu-18-04-authentication-error-on-loging-after-lock