i have enabled this by add the -l Parameter. Shutdown all LXC's, edit /lib/systemd/system/lxcfs.service, restart lxcfs and startup the containers again. Seems to work but i have lots of messages in syslog:
Apr 20 10:47:42 proxmox lxcfs[20932]: bindings.c: 5478: refresh_load: Open proc_path...