I’ve had this happen twice since upgrading from 24.04 to 26.04. To be clear, the symptoms were as described by Mihail in comment #12 above (and not whatever the OP reported):

loka 25 17:39:30 saegusa systemd[1]: Starting user@60579.service – User Manager for UID 60579…
loka 25 17:39:30 saegusa (systemd)[2446]: user@60579.service: Failed to determine credentials for user ’60579’: Unknown user
loka 25 17:39:30 saegusa (systemd)[2446]: user@60579.service: Failed at step USER spawning /usr/lib/systemd/systemd: Invalid argument
loka 25 17:39:30 saegusa systemd[1]: user@60579.service: Main process exited, code=exited, status=217/USER
loka 25 17:39:30 saegusa systemd[1]: user@60579.service: Failed with result ’exit-code’.
loka 25 17:39:30 saegusa systemd[1]: Failed to start user@60579.service – User Manager for UID 60579.
loka 25 17:39:30 saegusa systemd[1]: Started session-c1.scope – Session c1 of User gdm-greeter.
loka 25 17:39:30 saegusa systemd[1]: Started apache2.service – The Apache HTTP Server.
loka 25 17:39:30 saegusa /usr/libexec/gdm-wayland-session[2491]: dbus[2491]: Unable to set up transient service directory: XDG_RUNTIME_DIR ”/run/user/60579” is owned by uid 60579, not our uid 60578
loka 25 17:39:30 saegusa /usr/libexec/gdm-wayland-session[2491]: dbus-daemon[2491]: [session uid=60578 pid=2491 pidfd=5] Cannot set up inotify for ’/run/gdm3/home/gdm-greeter/.local/share/dbus-1/services’: Permission denied

The first time it happened during a normal reboot, and resolved itself after I rebooted again (using ssh to log in from another system). The second time (last night, as logged above) it happened after a reboot due to VLC causing an unrelated Xwayland crash, and no amount of rebooting ever fixed it.

I stopped nscd (`systemctl stop nscd; nscd -i passwd; nscd -i group`), after which restarting GDM (`systemctl restart gdm`) finally brought the login screen back. I then disabled nscd (`systemctl disable nscd`) to make sure it won’t mess things up again on the next reboot.

The suggestion to stop nscd came from ChatGPT, which apparently based it on a reply to an Ask Ubuntu thread about the same issue [1]. I don’t know what unwanted consequences disabling nscd potentially has, but at least I have a working desktop for now.

* [1] https://askubuntu.com/a/1570171