$ journalctl -b -u lightdm
Those messages were written on May 5th. Before the problem started, right?
Add the option -b to have the message for the last boot:
- [Trisquel-users] Why Has My Lock Screen/ Login Screen Turned Bla... davidpgil
- [Trisquel-users] Re : Why Has My Lock Screen/ Login Screen ... lcerf
- Re: [Trisquel-users] Why Has My Lock Screen/ Login Scre... davidpgil
- [Trisquel-users] Re : Why Has My Lock Screen/ Login... lcerf