メッセージ "Failed to start User Manager" がジャーナルに表示され、対応する "user@<UID>.service" ユニットは failed とマークされる

Issue

• ユーザーとしてシステムにログインするか、cron を実行すると、ジャーナルに次のメッセージが表示されます。

  [...] systemd[3994]: pam_unix(systemd-user:session): session opened for user rmetrich by (uid=0)
  [...] systemd[3994]: Failed to fully start up daemon: Permission denied
  [...] systemd[3995]: pam_unix(systemd-user:session): session closed for user rmetrich
  [...] systemd[1]: user@101166.service: Failed with result 'protocol'.
  [...] systemd[1]: Failed to start User Manager for UID 101166.
  [...] systemd[1]: Stopping /run/user/101166 mount wrapper...
  [...] sshd[3991]: pam_systemd(sshd:session): Failed to create session: Start job for unit user@101166.service failed with 'failed'
  

• user@<UID>.service ユニットを確認すると、failed と表示されます。

  # systemctl --failed 
  UNIT                LOAD   ACTIVE SUB    DESCRIPTION                
  ● user@101166.service loaded failed failed User Manager for UID 101166
  
  LOAD  =Reflects whether the unit definition was properly loaded.
  ACTIVE=The high-level unit activation state, i.e. generalization of SUB.
  SUB   =The low-level unit activation state, values depend on unit type.
  
  1 loaded units listed. Pass --all to see loaded but inactive units, too.
  To show all installed unit files use 'systemctl list-unit-files'.