After changing an user's password, I can no longer log in without going into "Other" and specifying the username *and* password

Whenever I let someone else work on my laptop, I have them use an account named “guest”. However, today, when I checked if it could be logged into, it wouldn’t work, I couldn’t log in. I went back to my main account and changed the password, but it still didn’t let me log in, until I went to the “Other” button in the login screen and specified the username and password manually. Then it let me log in.

Does anyone know what causes this or how I could solve this?

Have a look in the system journal for clues as to what happens when you attempt login.

Also you could try using plasmalogin manager instead of sddm.