After update, X11 login causes shutdown, Wayland works

I updated a few hours ago and during the update something triggered a systemd shutdown - apps closed, screens went black, keyboard became unresponsive and I had to power down.

On rebooting, SDDM looked normal, but as soon as I tried to log into an X11 session the same thing happened - immediate black screens and unresponsive keyboard - I couldn’t switch to a VT or anything.

On rebooting again, I tried my Plasma/Wayland session, and it works just fine - it’s what I’m posting from right now. I checked journalctl and I see a systemd shutdown triggering immediately on X11 login, but I have no idea why:

Mar 31 09:34:16 helios sddm[1494]: Authentication for user  "jonathan"  successful
Mar 31 09:34:16 helios sddm-greeter-qt6[1541]: Message received from daemon: LoginSucceeded
...
Mar 31 09:34:17 helios sddm-helper[1764]: Starting X11 session: "/usr/bin/X -nolisten tcp -background none -seat seat0 -noreset -keeptty -novtswitch -verbose 3" "/usr/share/sddm/scripts/Xsession \"/usr/bin/startplasma-x11\""
Mar 31 09:34:17 helios sddm-helper[1834]: Jumping to VT 2
Mar 31 09:34:17 helios sddm-helper[1834]: VT mode didn't need to be fixed
Mar 31 09:34:17 helios sddm[1494]: Session started true
...
Mar 31 09:34:17 helios sddm-helper-start-x11user[1834]: Running server: /usr/bin/X -nolisten tcp -background none -seat seat0 -noreset -keeptty -novtswitch -verbose 3 -auth /run/user/1000/xauth_eAbsvc -displayfd 13 vt2
Mar 31 09:34:27 helios systemd[1]: Stopping User Manager for UID 959...
Mar 31 09:34:27 helios systemd[1506]: Activating special unit Exit the Session...
Mar 31 09:34:27 helios systemd[1506]: Stopped target Main User Target.
Mar 31 09:34:27 helios systemd[1506]: Closed Socket to launch DrKonqi for a systemd-coredump crash.
Mar 31 09:34:27 helios wireplumber[1656]: stopped by signal: Terminated
Mar 31 09:34:27 helios systemd[1506]: Stopping Multimedia Service Session Manager...
Mar 31 09:34:27 helios wireplumber[1656]: disconnected from pipewire
Mar 31 09:34:27 helios systemd[1506]: Stopped Multimedia Service Session Manager.
Mar 31 09:34:27 helios systemd[1506]: Stopping PipeWire Multimedia Service...
Mar 31 09:34:27 helios systemd[1506]: Stopped PipeWire Multimedia Service.
Mar 31 09:34:27 helios systemd[1506]: Stopped target Basic System.
...
Mar 31 09:34:27 helios systemd[1506]: Reached target Shutdown.
Mar 31 09:34:27 helios systemd[1506]: Finished Exit the Session.
Mar 31 09:34:27 helios systemd[1506]: Reached target Exit the Session.
...
Mar 31 09:34:28 helios systemd[1]: systemd-hostnamed.service: Deactivated successfully.
Mar 31 09:34:29 helios systemd[1]: systemd-localed.service: Deactivated successfully.
Mar 31 09:34:31 helios systemd-logind[1372]: Power key pressed short.

I’ve checked, and none of my X config files changed, and it happens with a new user, too. The update did include the kernel, libx11 and wireplumber and a bunch of other stuff that get used in login…but my Wayland session works fine so my kernel modules and whatnot should be good.

I can provide the whole unabridged log, but I didn’t want to post too much. I’m just not sure where to continue the troubleshooting.

I’m just at a loss of what to do next. I manually reverted every single package in the update (which was a bit over 100, it’d been a week or so), and it still dies when trying to start my Plasma/X11 session.

Plasma/Wayland still works fine.

I can rebuild the system - would have been faster than troubleshooting - but I really want to understand what happened/what’s causing this.

If I go to a VT from the greeter and just try ‘startx’, a basic X server does start on one of my screens, with a couple xterms I can close and drop back to the VT.

But trying to launch my Plasma/X11 or my old Budgie/X11 sessions blank the screens and lock all input aside from the power button, which does still trigger a systemd powerdown event so at least I can shut down cleanly.

Nothing in the logs except what I posted above.