"Authentication failure" attempting to log in

[Thad]

Hello,

My problem appears similar to the ones described in these threads:

Cannot login through DM, "Authentication Failure" on root/user terminal login

Greetings! After restarting my computer I get to my display manager as normal but the wallpaper is not my personal one (looks like a manjaro default wallpaper) and when I attempt to login as my regular user, the screen goes black thereafter. Following this, I attempted to login through terminal (tty2). When attempting to login both root/regular user say, "Authentication Failure". I also have finger print (fprint) set on my root and user accounts. If I swipe the finger print wrong it eventually…

Manjaro KDE boots to black screen with only cursor.

Hi, it looks like my manjaro broke again. It's all of the sudden, i know this because i am streaming a movie from my pc to TV and i lost connection. I moved the mouse but screen is dead, so i just rebooted and now i only get a blank screen with cursor. command line works(Alt + Ctrl + F2) but i cannot login to my account. something strange - when i use the right password it shows 'Authentication Failure' and refreshes itself. when i use wrong password it just says 'login incorrect' I can lo…

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?

Thanks.

[Thad]

I've fixed it.

On booting to LiveUSB, I discovered that my root partition was full. I resized my partitions and gave it another 10GB, then rebooted and was able to log in without issue (LightDM came right up too and I was able to log into KDE).

Turned out to be a simple fix, but hopefully this will be useful in case anyone else has the same issue.

(And now to figure out why my root partition is full; I thought 30GB would be enough.)