After a reboot of a physical test server, two out of 5 Linux Containers (LXC) didn't start up automatically anymore.
When I manually tried to start them, I got the following error:
lxc-start: command get_cgroup failed to receive response
Although my research on the web pointed me to an Apparmor bug (Ubuntu bug #1295774), I could rule this bug out because the "fixed" Apparmor version was already installed:
dpkg -l | grep appa
ii apparmor 2.8.95~2430-0ubuntu5 amd64 User-space parser utility for AppArmor
ii libapparmor-perl 2.8.95~2430-0ubuntu5 amd64 AppArmor library Perl bindings
ii libapparmor1:amd64 2.8.95~2430-0ubuntu5 amd64 changehat AppArmor library
Interestingly, as I mentioned at the begin, other LXC's were started without problem. I checked out the config files and found a difference that the started containers were using the direct path of a logical volume (LV) as rootfs while the other two (which didn't start) were using a directory path.
Turns out... this path was not mounted (I forgot the entry in /etc/fstab). ^^
After mounting the LV's to the expected path, lxc-start worked fine.
So the error message "get_cgroup failed to receive response" can also appear if the rootfs is missing or not mounted.