My system went into some type of boot loop, a flashing black/dark grey screen. While the error message just flashed, it indicated an out of memory error and systemd stopped the lightdm service.
Lightdm had just been updated to lightdm-1:1.28.0-1.1. Reverting back to lightdm-1:1.28.0-1 fixed the boot problem.
I am running linux4.20.7-1 and systemd 239