Gfilui

I updated a machine to Kubuntu 18.04 by a complete reinstall. We still have user management using NFS/NIS. Packages are installed, users can log in. I have one local user dhlocal (userid: 1000) with home directory /localhome/dhlocal and users like dh (userid: 5000) with home dir /home/dh.

I can log in dhlocal to the X/KDE session and work there without any problems - if only that user is logged in.

If I log in dh, the system pauses for some 20 seconds (regardless whether the login is on console or X). Then, KDE comes up. After about two minutes the system suddenly crashes. .xsession-errors at the end looks like this:

The session is not registered: "PID 11214 does not belong to any known session"
QObject::connect: No such signal 
QDBusAbstractInterface::PrepareForSleep(bool)
powerdevil: Backend is ready, KDE Power Management system initialized
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-dh'
klauncher: Exiting on signal 15
ICE default IO error handler doing an exit(), pid = 11236, errno = 32
The X11 connection broke: I/O error (code 1)
XIO: fatal IO error 4 (Unterbrechung während des Betriebssystemaufrufs) on X server ":0"
 after 37 requests (37 known processed) with 0 events remaining.

The weird thing is: If I log in dh on a console (i.e. Ctrl-Alt-F2 and login) while having dhlocal logged in to X/KDE, the dhlocal session also crashes after some two minutes. This even happens if I log in again dhlocal on KDE as long as dh is logged in on any console. Messages in .xsession-errors of dhlocal:

The X11 connection broke (error 1). Did the X11 server die?
kdeinit5: sending SIGTERM to children.
kdeinit5: Exit.
klauncher: Exiting on signal 15
The X11 connection broke (error 1). Did the X11 server die?
The X11 connection broke: I/O error (code 1)
XIO: fatal IO error 2 (Datei oder Verzeichnis nicht gefunden) on X server ":0"
 after 3523 requests (3523 known processed) with 0 events remaining.
klauncher: Exiting on signal 1
The X11 connection broke: I/O error (code 1)
XIO: fatal IO error 2 (Datei oder Verzeichnis nicht gefunden) on X server ":0"
 after 1122 requests (1122 known processed) with 0 events remaining.
QProcess: Destroyed while process ("kwin_x11") is still running.
The X11 connection broke (error 1). Did the X11 server die?
Unexpected response from KInit (response = 0).
The X11 connection broke (error 1). Did the X11 server die?
startkde: Could not start ksmserver. Check your installation.
polkit-agent-helper-1: pam_authenticate failed: Authentication failure
Error: Can't open display: :0
Could not connect to D-Bus server: 
org.freedesktop.DBus.Error.NoServer: 
Failed to connect to socket /tmp/dbus-V6z3BsURMt: Verbindungsaufbau 
 abgelehnt
startkde: Shutting down...
kdeinit5_wrapper: Warning: connect(/run/user/1000/kdeinit5__0) failed: : 
No such file or directory
Error: Can not contact kdeinit5!
xprop: unable to open display ':0'
xprop: unable to open display ':0'
startkde: Done.

I removed the complete environment of dh and replaced .profile/.bashrc with dhlocal's version. I observe two things with dh:

. Logging in has this 20+ seconds delay which happens even before any bash startup file in $HOME or /etc is executed. I do not know where this comes from.

. For some reason, XDG_RUNTIME_DIR is not set for dh. Other environment variables, however, are. Once again .xsession-errors for dh:

Xsession: X session started for dh at Mo 30. Apr 17:06:17 CEST 2018
localuser:dh being added to access control list
dbus-update-activation-environment: systemd --user not found, ignoring --systemd argument
dbus-update-activation-environment: setting QT_ACCESSIBILITY=1
dbus-update-activation-environment: systemd --user not found, ignoring --systemd argument
dbus-update-activation-environment: setting LANG=de_DE.UTF-8
dbus-update-activation-environment: setting DISPLAY=:0
dbus-update-activation-environment: setting PAM_KWALLET5_LOGIN=/tmp/kwallet5_dh.socket
dbus-update-activation-environment: setting MANDATORY_PATH=/usr/share/gconf/plasma.mandatory.path
dbus-update-activation-environment: setting USER=dh
dbus-update-activation-environment: setting DESKTOP_SESSION=plasma
dbus-update-activation-environment: setting DEFAULTS_PATH=/usr/share/gconf/plasma.default.path
dbus-update-activation-environment: setting PWD=/home/dh
dbus-update-activation-environment: setting HOME=/home/dh
dbus-update-activation-environment: setting QT_ACCESSIBILITY=1
dbus-update-activation-environment: setting XDG_SESSION_TYPE=x11
dbus-update-activation-environment: setting XDG_DATA_DIRS=/usr/share/plasma:/usr/local/share:/usr/share:/var/lib/snapd/desktop
dbus-update-activation-environment: setting XDG_SESSION_DESKTOP=KDE
dbus-update-activation-environment: setting SHELL=/bin/bash
dbus-update-activation-environment: setting XDG_SESSION_CLASS=user
dbus-update-activation-environment: setting XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
dbus-update-activation-environment: setting IM_CONFIG_PHASE=1
dbus-update-activation-environment: setting XDG_CURRENT_DESKTOP=KDE
dbus-update-activation-environment: setting GPG_AGENT_INFO=/home/dh/.gnupg/S.gpg-agent:0:1
dbus-update-activation-environment: setting SHLVL=1
dbus-update-activation-environment: setting LOGNAME=dh
dbus-update-activation-environment: setting DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-rMq6WSod2U,guid=9e31faa7d68efa98592ebb2b5ae730ea
dbus-update-activation-environment: setting PAM_KWALLET_LOGIN=/tmp/kwallet_dh.socket
dbus-update-activation-environment: setting XAUTHORITY=/home/dh/.Xauthority
dbus-update-activation-environment: setting XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session3
dbus-update-activation-environment: setting XDG_CONFIG_DIRS=/etc/xdg/xdg-plasma:/etc/xdg:/usr/share/kubuntu-default-settings/kf5-settings
dbus-update-activation-environment: setting PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
dbus-update-activation-environment: setting _=/usr/bin/dbus-update-activation-environment

What's going on here? How can I fix this? Is any further information needed?