According to https://github.com/lxc/lxd/issues/2937 : Building with cgmanager does not work for systemd-based containers. all cgmanager occurences should be removed.
Fixed as of 2.11. Thanks for linking the issue thread - I knew cgmanager would be deprecated but it was good to have upstream's confirmation to get rid of it. Consider installing lxcfs per stephane's comment, if you haven't already. It makes uptime and /proc/cpuinfo show guest-specific values not the host values.