My problem appears similar to the ones described in these threads:
I'm running Manjaro KDE with nVidia proprietary drivers. I ran updates and rebooted. Startup goes to a black screen with a mouse pointer (which I can move); wallpaper, login prompt, etc. never appear.
If I drop to a VT with Ctrl-Alt-F2 and try to log in, I get "Authentication error." This happens when I enter my username and password correctly, and is distinct from the error I get when I type an incorrect username or password (that gives me the "Login incorrect" error). This occurs when I try to log in as my user account or as root.
First question: what can I do to get my login working again so I can troubleshoot the issue further? I can boot from LiveUSB and should be able to get into my system from there. I'm using btrfs and there should be a snapshot of my system state before I ran the update; is my best first step to run snapper --undochange? If so, how can I do that from a live boot? Should I use chroot, or just install snapper into the live environment, or is there something else I should try?