After a fresh install of Juno, things were mostly working with the default open source nouveau driver. I installed the nvidia-340 driver using the App Center, and now the lightdm-greeter is completely broken. I chose auto-login, so I can get to my desktop. Incidentally, the "glowing e" splash does not display, and nvidia logo flashes for a few microseconds, then I get to my desktop. If I try to lock the screen, log out, or change user (all which use lightdm-greeter), I am stuck on a black screen.
I can ssh into the machine and
sudo killall lightdm
and that gets me back to the desktop.
I would really like to get lightdm working with the nvidia drivers. Any idea where I should look? I reverted back to nouveau for now.