A stop job is running for Session c2 of user

The following message appears almost every time I shutdown my computer:

A stop job is running for Session c2 of user ... (1min 30s)

It waits for 1min30s then continues the shutdown process. I follow this systemd shutdown diagnosis guide and get the shutdown-log.txt (I can't paste directly the log here because it's very long). Unfortunately, I don't understand the log by myself. Could anyone help me to find out what makes my system doesn't shutdown properly?

I run Arch Linux with kernel 4.4.5-1-ARCH, my systemd version is 229-3.

Addition 1: I observe that every time I logout, and then shutdown my computer from the login screen, it doesn't get the message A stop job is running.... I tried to logout before shutdown for many times, so I think it doesn't occur by chance. Hope that information could help.

Addition 2: It is always session c2 that causes shutdown hanging. So as @n.st suggest, I looked at Diagnosing Shutdown Problems again and stored loginctl session-status c2 instead of dmesg, but then there is nothing on the shutdown-log.txt. I replaced loginctl session-status c2 by systemd-cgls and got the following log:

Control group /:
-.slice
└─init.scope
  ├─   1 /usr/lib/systemd/systemd-shutdown reboot --log-level 6 --log-target ...
  ├─1069 /usr/lib/systemd/systemd-shutdown reboot --log-level 6 --log-target ...
  ├─1071 /bin/sh /usr/lib/systemd/system-shutdown/debug.sh reboot
  └─1074 systemd-cgls

Any ideas?

Note: After I updated to kernel 4.6.4-1-ARCH and systemd 230-7, the error no longer happened.