Can't login via lightdm

I am using Ubuntu 18.04 that is setup for remote logins via XDMCP/LightDM. Authentication is done via LDAP/SSSD. Everything works fine and users can login into KDE Plasma, except that for some users the session is immediately ended.

I checked the permissions on .Xauthority files – which are empty by the way, just like .xsession-errors.

The users that can not login via LightDM – can log in via terminal just fine. None of /var/log/syslog, /var/log/lightdm or /var/log/auth.log gives me any hints. AppArmor and SeLinux are disabled.

From /var/log/auth.log:

Apr 29 14:05:26 desktop-ubuntu systemd[1]: session-c79.scope: Killing process 26623 (lightdm) with signal SIGTERM.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: session-c79.scope: Killing process 26626 (lightdm-greeter) with signal SIGTERM.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: session-c79.scope: Killing process 26627 (lightdm-gtk-gre) with signal SIGTERM.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Stopping Session c79 of user lightdm.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Stopped Session c79 of user lightdm.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Created slice User Slice of XXX.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Starting User Manager for UID 1406...
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Started Session c80 of user XXX.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on GnuPG cryptographic agent and passphrase cache.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on GnuPG cryptographic agent and passphrase cache (restricted).
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Started Pending report trigger for Ubuntu Report.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on GnuPG cryptographic agent (ssh-agent emulation).
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on GnuPG network certificate management daemon.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Reached target Timers.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Reached target Paths.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on GnuPG cryptographic agent and passphrase cache (access for web browsers).
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Starting D-Bus User Message Bus Socket.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Listening on D-Bus User Message Bus Socket.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Reached target Sockets.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Reached target Basic System.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Reached target Default.
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Startup finished in 40ms.
Apr 29 14:05:26 desktop-ubuntu systemd[1]: Started User Manager for UID 1406.
Apr 29 14:05:26 desktop-ubuntu lightdm[25089]: Error activating login1 session: GDBus.Error:org.freedesktop.DBus.Error.NotSupported: Operation not supported
Apr 29 14:05:26 desktop-ubuntu systemd[26668]: Started D-Bus User Message Bus.
Apr 29 14:05:26 desktop-ubuntu dbus-daemon[26706]: [session uid=1406 pid=26706] AppArmor D-Bus mediation is enabled