I cannot reliably reproduce this problem and do not really have any idea what might be triggering it.  Maybe it's situations where resources (CPU, HD) are tight.  While I have come across it a couple of times in the past I have yet to experience it again after reporting this ticket.

I have lxc installed but do not actively use it.

The content of the file you requested is not time-stamped, so I don't know if what's in there has anything to do with what I reported here.  It might be from a completely different time.  OK, here it is.

$ sudo cat /var/log/upstart/cgmanager.log
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:remove_main: Failed to remove /run/cgmanager/fs/none,name=systemd/user/1000.user/c2.session: Device or resource busy
cgmanager:remove_main: Failed to remove /run/cgmanager/fs/none,name=systemd/user/1000.user: Device or resource busy
cgmanager: Invalid path /run/cgmanager/fs/none,name=systemd///user (No such file or directory)
cgmanager:list_children_main: Could not determine the requested cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:remove_main: Failed to remove /run/cgmanager/fs/none,name=systemd/user/1000.user/c2.session: Device or resource busy
cgmanager:remove_main: Failed to remove /run/cgmanager/fs/none,name=systemd/user/1000.user: Device or resource busy
cgmanager: Invalid path /run/cgmanager/fs/none,name=systemd///user (No such file or directory)
cgmanager:list_children_main: Could not determine the requested cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager: Invalid path /run/cgmanager/fs/none,name=systemd///user (No such file or directory)
cgmanager:list_children_main: Could not determine the requested cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup
cgmanager:get_pid_cgroup_main: Could not determine the requestor cgroup