Mmm interesting.
Your login screen is set to display the same background as selected user, it means the user’s desktop background. What is that file and is it accessible to root for lightdm to display it ? Is your personal folder encrypted ?
Reminds me of [Request] A default way for changing wallpaper every few minutes
background=/usr/share/backgrounds/Xplo_by_Hugo_Cliff.png
↑ this, is a default setting as stored in dconf keys, but overiden by /etc/lightdm/slick-greeter.conf
Mine for exemple contains :
[Greeter]
draw-grid=true
draw-user-backgrounds=true
background=
show-a11y=false
show-power=false
theme-name=Adapta
icon-theme-name=Papirus-Dark
activate-numlock=true
Whilst dconf settings regarding x.dm.slick-greeter are different :
coeur-noir@asgard:~$ gsettings list-recursively x.dm.slick-greeter
x.dm.slick-greeter group-filter @as []
x.dm.slick-greeter hidden-users @as []
x.dm.slick-greeter xft-dpi 96.0
x.dm.slick-greeter show-quit true
x.dm.slick-greeter show-hostname true
x.dm.slick-greeter play-ready-sound ''
x.dm.slick-greeter enable-hidpi 'auto'
x.dm.slick-greeter theme-name 'Pocillo-dark'
x.dm.slick-greeter other-monitors-logo ''
x.dm.slick-greeter font-name 'Ubuntu 11'
x.dm.slick-greeter activate-numlock true
x.dm.slick-greeter xft-hintstyle 'hintslight'
x.dm.slick-greeter background '/usr/share/backgrounds/Xplo_by_Hugo_Cliff.png'
x.dm.slick-greeter draw-user-backgrounds true
x.dm.slick-greeter icon-theme-name 'gnome'
x.dm.slick-greeter logo ''
x.dm.slick-greeter show-power true
x.dm.slick-greeter draw-grid false
x.dm.slick-greeter screen-reader false
x.dm.slick-greeter show-a11y true
x.dm.slick-greeter show-clock true
x.dm.slick-greeter onscreen-keyboard false
x.dm.slick-greeter show-keyboard true
x.dm.slick-greeter xft-rgba 'rgb'
x.dm.slick-greeter background-color '#000000'
x.dm.slick-greeter high-contrast false
x.dm.slick-greeter xft-antialias true
coeur-noir@asgard:~$
( here I had manually changed values in dconf for draw-user-backgrounds, activate-numlock )
So you should check what’s in your /etc/lightdm/slick-greeter.conf as this is the file written by the login screen utility which overrides default lightdm conf.