LTSP clients do not poweroff/reboot in Ubuntu 15.10

[Fotis Tsamis]

Distribution: Ubuntu Mate 15.10
Testing Environment: Thin client running on Virtualbox
Description: When selecting Shutdown from the LDM menu or executing poweroff in SCREEN_02=shell the shutdown procedure starts, but it never ends.

Pressing F1 shows (the last part of messages):

[OK] Stopped Create Volatile Files and Directories.
[OK] Stopped target Local File Systems.
         Unmounting /rofs...
         Unmounting /cow...
[OK] Stopped Load/Save Random Seed.
[OK] Unmounted /rofs...
[OK] Unmounted /cow...
[OK] Reached target Unmount All Filesystems.
[OK] Reached target Local File Systems (Pre).
[OK] Stopped Create Static Device Nodes in /dev.
[OK] Stopped Remount Root and Kernel File Systems.
[OK] Reached target Shutdown.

After a couple of minutes the messages below show up and get repeated every 2 minutes:

INFO: task systemd:1 blocked for more than 120 seconds.
           Tainted: G OE 4.1.0-3-generic #3-Ubuntu
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.