password restrictions are different in the GUI and on the command line.

[Ralf Mardorf]

On Sun, 17 Jul 2016 17:47:24 +0200, s.achterop at rug.nl wrote:
>The password of the main user set in the installation is something
>like:  abc1234 The password of another user set by adduser is
>something like: 12_abcdefgh These passwords work with a console or ssh
>login.
>
>With these passwords lightdm gives an error about "incorrect
>password ... " The lightdm logfile says:
>     [+731.48s] DEBUG: Authenticate result for user thisuser:
> Authentication failure
>
>So for me lightdm doesn't work with simple passwords.
>By the way, it started working when I made the passwords longer.

Did you make the passwords longer or did you use different chars too?

Chris' hint IMO is very good, it could happen that lightdm does use
another keyboard layout.