According to the syslog, it seemed that lightdm quit the user session and the xserver once the user logged in.
Jul 9 13:56:21 wv01 lightdm[1054]: Error activating login1 session: GDBus.Error:org.freedesktop.DBus.Error.NotSupported: Operation not supported
Jul 9 13:56:21 wv01 kernel: [484381.398773] audit: type=1400 audit(1688925381.677:8532): apparmor="ALLOWED" operation="open" profile="/usr/sbin/sssd" name="/proc/124010/cmdline" pid=1021 comm="sssd_nss" requested_mask="r" denied_mask="r" fsuid=0 ouid=0
Jul 9 13:56:23 wv01 systemd[1]: session-c20.scope: Deactivated successfully.
Jul 9 13:56:23 wv01 systemd[1]: session-c20.scope: Consumed 1.209s CPU time.
Jul 9 13:56:23 wv01 org.a11y.Bus[172050]: X connection to :104 broken (explicit kill or server shutdown).
I found a similar question from stackoverflow, see https://askubuntu.com/questions/1139133/cant-login-via-lightdm, from its comments, I suggest you try the following things:
1. Add user-authority-in-system-dir=true to the LightDM config
2. Check whether the user's HOME directory is full with the command
df -h $HOME
