Comment 7 for bug 1796374

I do not know if this error message is related to your issue. Please attach the full log file(s) to make sure that we do not miss anything relevant.

Regardless of that, I would suggest to test with another greeter:

- install unity-greeter (sudo apt install --no-install-recommends unity-greeter)
- move or rename the file /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
- reboot or restart lightdm

Furthermore, you could boot into the live session of the Xubuntu 18.10 development release, and check if the problem is still present. Note that light-locker needs to be manually started.