Comment # 29 on bug 1156501 from Dr. Werner Fink

Ohmm ... 

 systemctl --user status dbus.socket
 * dbus.socket - D-Bus User Message Bus Socket
    Loaded: loaded (/usr/lib/systemd/user/dbus.socket; static; vendor preset:
disabled)
    Active: inactive (dead)
    Listen: /run/user/223/bus (Stream)

... that seems to be a part of the problem, but dbus.socket is linked below
/usr/lib/systemd/user/sockets.target.wants/

Also set debug on pam_systemd.so in common-session to see more:

 Nov 15 10:51:01 noether sddm-helper[23902]: [PAM] Starting...
 Nov 15 10:51:01 noether sddm-helper[23902]: [PAM] Authenticating...
 Nov 15 10:51:01 noether sddm-helper[23902]: gkr-pam: unable to locate daemon
control file
 Nov 15 10:51:01 noether sddm-helper[23902]: [PAM] Preparing to converse...
 Nov 15 10:51:01 noether sddm-helper[23902]: pam_kwallet5(sddm:auth): (null):
pam_sm_authenticate
 Nov 15 10:51:01 noether sddm-helper[23902]: [PAM] Conversation with 1 messages
 Nov 15 10:51:01 noether sddm-helper[23902]: [PAM] returning.
 Nov 15 10:51:01 noether sddm[2224]: Authenticated successfully
 Nov 15 10:51:01 noether sddm-greeter[23827]: Message received from daemon:
LoginSucceeded
 Nov 15 10:51:01 noether sddm-helper[23902]: pam_kwallet5(sddm:setcred):
pam_kwallet5: pam_sm_setcred
 Nov 15 10:51:01 noether sddm-helper[23902]: pam_systemd(sddm:session):
pam-systemd initializing
 Nov 15 10:51:01 noether sddm-helper[23902]: pam_systemd(sddm:session): Asking
logind to create session: uid=223 pid=23902 service=sddm type=x11 class=user
desktop=KDE seat=seat0 vtnr=7 tty= display=:0 remote=no remote_user=
remote_host=
 Nov 15 10:51:01 noether sddm-helper[23902]: pam_systemd(sddm:session): Session
limits: memory_max=n/a tasks_max=n/a cpu_weight=n/a io_weight=n/a
 Nov 15 10:51:01 noether systemd[1]: Created slice User Slice of UID 223.
 Nov 15 10:51:01 noether systemd[1]: Starting User Runtime Directory
/run/user/223...
 Nov 15 10:51:01 noether systemd-logind[2163]: New session 71 of user werner.
 Nov 15 10:51:01 noether systemd[1]: Started User Runtime Directory
/run/user/223.
 Nov 15 10:51:01 noether systemd[1]: Starting User Manager for UID 223...
 Nov 15 10:51:01 noether sddm-helper[23819]: [PAM] Closing session
 Nov 15 10:51:01 noether sddm-helper[23819]: pam_unix(sddm-greeter:session):
session closed for user sddm
 Nov 15 10:51:01 noether sddm-helper[23819]:
pam_kwallet5(sddm-greeter:session): pam_kwallet5: pam_sm_close_session
 Nov 15 10:51:01 noether sddm-helper[23819]: [PAM] Ended.
 Nov 15 10:51:01 noether sddm[2224]: Auth: sddm-helper exited successfully
 Nov 15 10:51:01 noether sddm[2224]: Greeter stopped.
 Nov 15 10:51:01 noether systemd-logind[2163]: Session 70 logged out. Waiting
for processes to exit.
 Nov 15 10:51:01 noether systemd[23907]: pam_systemd(systemd-user:session):
pam-systemd initializing
 Nov 15 10:51:01 noether systemd[23907]: pam_unix(systemd-user:session):
session opened for user werner(uid=223) by (uid=0)
 Nov 15 10:51:01 noether systemd[23907]: pam_kwallet5(systemd-user:session):
(null): pam_sm_open_session
 Nov 15 10:51:01 noether systemd[23907]: pam_kwallet5(systemd-user:session):
pam_kwallet5: not a graphical session, skipping. Use force_run parameter to
ignore this.
 Nov 15 10:51:01 noether systemd[23907]: Reached target Paths.
 Nov 15 10:51:01 noether systemd[23907]: Reached target Timers.
 Nov 15 10:51:01 noether systemd[23907]: dbus.socket: Socket service
dbus.service not loaded, refusing.
 Nov 15 10:51:01 noether systemd[23907]: Failed to listen on D-Bus User Message
Bus Socket.

Indeed:

 systemctl --user status dbus.service
 Unit dbus.service could not be found.