Comment # 3 on bug 1012883 from Franck Bui

If you use persistent journals (probably the default on TW), you should be able
to read the old logs from previous boots(see man journalctl and the '-b'
option).

Otherwise I would recommend to switch to persistent journal mode (by creating
/var/log/journal directory and restart systemd-journald.service).

Could you try the following test (several time if you can't reproduce):

 0/ enable the debug logs: "systemd-analyze set-log-level debug"

 1/ try to login (as a regular user) via your display manager (GDM, LXDM, ...)
and start all applications you use to execute (chromium, firefox, ...).

 2/ open a root session (via ssh for example)

 3/ in your root session: retrieve the session number created when you logged
in as a regular user by executing "loginctl"

 4/ in your root session: stop your regular session with "systemctl stop
session-<id>.scope" where "id" is the session number you found in the previous
step.

 5/ if the command is stuck, this means that you reproduced the issue otherwise
back to step 1/

 6/ so the command is stuck which probably means that a process is failing to
react to the SIGTERM sent by systemd when you stopped/closed your user session.
In that case please show the content of the following commands:

   6.1/ ps aux
   6.2/ systemd-cgls
   6.3/ journalctl -b

Thanks !