Comment # 10 on bug 1012883 from Kyaw Thu Soe

Created attachment 705719 [details]
multiple

By setting permanent debug LogLevel, I got additional messages between the time
lapse. The problem is always before the "session-1.scope: Stopping timed out.
Killing." line.

If you search my attachment the keyword "timed out" several time FROM THE
BOTTOM, you will found the offending process, systemd-timesyncd.service which
is not previously found due to the info LogLevel. Every other previous hanging
can be browsed with the "timed out" keyword or "session-1.scope: Stopping timed
out. Killing." keyword. It's always after the "Received SIGRTMIN+20 from PID
2332 (plymouthd)" and before the "session-1.scope: Stopping timed out".
If you query the keyword "timed out" several times, there's so much occurrence
of the line 
"systemd-timesyncd[1077]: Timed out waiting for reply from 216.239.35.12:123
(time4.google.com)" here and there which is highly distrust-able.

One coredump occurence just before the problem is also found:

"systemd-coredump[2328]: Process 2327 (klauncher) of user 1000 dumped core.
Nov 29 20:17:09 linux-ghot systemd[1]: Received SIGRTMIN+20 from PID 2332
(plymouthd)."

According to this bug report:
https://www.reddit.com/r/archlinux/comments/4bawf7/a_stop_job_is_running_for_session_c2_of_user/
it's due to timesyncd.

According to https://github.com/systemd/systemd/issues/1615 and
https://github.com/systemd/systemd/issues/2691

it's due to coredump which was fixed at the later versions.