It seems that adding a new user through "System Settings" -> "Account Details" -> "New User" doesn't work: I fill in the new user's details and when I click "Apply" it turns back to grey and nothing happens. Running system settings from a console, it gives the following output > systemsettings5 log_user_manager: 1000 log_user_manager: Checking password log_user_manager: Verify password is empty log_user_manager: Checking password log_user_manager: Verify password is empty log_user_manager: Checking password log_user_manager: Quality: 31 log_user_manager: Saving on Index: 1 > No sure when it first appeared. I just noticed it in KDE Neon user edition having latest packages installed (as of December 18th).
*** This bug has been marked as a duplicate of bug 373276 ***