Haven't seen it in a few days. I'll reboot and see if I can reproduce it. It usually happens after rebooting the host, when launching new containers or existing ones would autostart.

Info you requested. I think the /usr/share/lxc/... might have been a red herring. I'm exclusively using LXD on this xenial host, not messing with the old lxc commands.

$ dpkg -l lxc liblxc1 lxd lxd-client lxcfs
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                            Version              Architecture         Description
+++-===============================-====================-====================-====================================================================
ii  liblxc1                         2.0.0~rc5-0ubuntu1   amd64                Linux Containers userspace tools (library)
ii  lxc                             2.0.0~rc5-0ubuntu1   all                  Transitional package for lxc1
ii  lxcfs                           2.0.0~rc2-0ubuntu2   amd64                FUSE based filesystem for LXC
ii  lxd                             2.0.0~rc1-0ubuntu3   amd64                Container hypervisor based on LXC - daemon
ii  lxd-client                      2.0.0~rc1-0ubuntu3   amd64                Container hypervisor based on LXC - client

$ lxc info
apicompat: 0
auth: trusted
environment:
  addresses:
  - 192.168.88.234:8443
  - 192.168.122.1:8443
  - 10.0.3.1:8443
  architectures:
  - x86_64
  - i686
  certificate: |
    -----BEGIN CERTIFICATE-----
    MIIFqjCCA5KgAwIBAgIQXpne6Qjwhg8de+RmyV1+mTANBgkqhkiG9w0BAQsFADA6
    MRwwGgYDVQQKExNsaW51eGNvbnRhaW5lcnMub3JnMRowGAYDVQQDDBFyb290QG1h
    d2hyaW4tc2tlbDAeFw0xNjAyMjgwMzEyMzdaFw0yNjAyMjUwMzEyMzdaMDoxHDAa
    BgNVBAoTE2xpbnV4Y29udGFpbmVycy5vcmcxGjAYBgNVBAMMEXJvb3RAbWF3aHJp
    bi1za2VsMIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEAzv/3uX3JWduq
    wmtbyTABmfJkup6Z5Lh4lKPXgL/H2gQ/mlccORKm1eDZhAGmv9UuQGeMRHEneJqD
    V9c3f7/9cJBwvz2loKlppWj0ohAzTP91L8paeUSfP4X9EAr702Qjyb2ig+xWv5tM
    cJxbdl0zGpjYO1P+xmUdthSidsFrzWQPXptOlcZvak7n0QL5GlkVXdqX+she2Pbs
    ONtyTaBSpF3zEYv6cM9ZeJYL4Hl7LEQ1/p8ojpOyaxO8B1Cn/gIbuDqgzRwmei90
    Aca06YDF4SHVcl8qFajrwkPF3jWW5pgS8sAJlYoq2+ROhl0CnpdBl4AiJrvfFsIs
    RL8dKSuFA6AcLhYooWgMy6UWR8mLbmYHp04ThuBDoRaTt0uGLDlTAfMg7e8Gwpz+
    aEwxSzzQhvJYr4e6TSP1C4zXNqS5mUHfm6RtfEccVFmq6vyqZGELAyREce76J88V
    FMf/V+KYlQYUxo0JH2k+BYMO4Iigar0+8p8o8drqh6Lks5zTP6idsKa0LSWA4rwm
    3+5hjnVJtFa9CVCItJW3r0+nczwtAbjvRojkr7Yb2Kifufhilb+I695qSk+Toug3
    HKOODTqc9sbH+urmfr7jBBexACtWVX/tMptWFnBmcqoN4ptSZutY7CPf+KR//9p0
    8fFRQP+ItmElJHRFO+madGOBMVrC7j8CAwEAAaOBqzCBqDAOBgNVHQ8BAf8EBAMC
    BaAwEwYDVR0lBAwwCgYIKwYBBQUHAwEwDAYDVR0TAQH/BAIwADBzBgNVHREEbDBq
    ggxtYXdocmluLXNrZWyCETE5Mi4xNjguODguMjM0LzI0ghxmZTgwOjozZWE5OmY0
    ZmY6ZmU1Njo0N2RjLzY0ggsxMC4wLjMuMS8yNIIcZmU4MDo6NThkNTpjMmZmOmZl
    ZTQ6NzJiYy82NDANBgkqhkiG9w0BAQsFAAOCAgEAB6aFItuxlZm5+2/IB2eCVAM0
    eQbO6dfvfF2khfiEbWWaKPtkZSYKlDIcoOph35obnNMQjT+y4zlnF/fepvjq8P1R
    yGd+Q+GMcXWVRht3uIMW2ZqwNqOujunyn9+Hl1SYi1dV1g/CH9lJt8I7FKIvyieh
    siZRe5Zp7TdPREkIJveuz8qB3X87WVh9bqvMpoX91Mgrjzd3qATef/tN0HP+b26Y
    XjtRz9rhOkUfJx9b5HsyBLuGOQS979twl9LPKc1WsIrJaavY1JBT7SIwX3W7u7CU
    UWueEQoVvA9pkgeL8NXYYoHOkkrfW3oqExkuA1pqucMt8iRjn88njV57Vk576tjZ
    +bNqLL6R4xoUvnr9VIwq7tMLvodK/E5WvsmP5GrdMru2sa/xd+fS0137oOYqP4Qb
    T296jMcehHg5Gg1ZR1DbTqzWFPKb+CtXU+bqI68Hplq5Lemufsman2C2dthmKwcm
    OumpykWiH9vNKauY52VR8iffppBsH7jup1+yVtIOiT4Whp5LGBtZdYri7GE77qv+
    qtBL7HdeBJxsBadRIfjdeLZlZPlMX7hV+v8Bzxz9fGuPKSnxDdpoPw1XF+zP1YAn
    VeDCRo7oCFV40nVFyb5wCreDoegtSFuQt7/Guv9u/gQSfmGoY1QyjlaHh2pJAAgS
    CPRGObnrdrQMMEqLDaI=
    -----END CERTIFICATE-----
  driver: lxc
  driverversion: 2.0.0.rc5
  kernel: Linux
  kernelarchitecture: x86_64
  kernelversion: 4.4.0-10-generic
  server: lxd
  serverpid: 2382
  serverversion: 2.0.0.rc1
  storage: zfs
  storageversion: "5"
config:
  core.https_address: '[::]'
  storage.zfs_pool_name: lxdpool
public: false