lxc-start-ephemeral stops working with kernel 3.18 - overlayfs change?

With the previous vivid kernel 3.16.0-28-generic,"lxc-start-ephemeral -o containername" works fine. With the current 3.18.0-8-generic it fails with "the container failed to start". This is reproducible in a clean cloud VM:

$ sudo apt install lxc
$ sudo lxc-create -t busybox -n c1

# works fine under all kernels:
$ sudo lxc-start -n c1 -F

$ sudo lxc-start-ephemeral -o c1
The container 'c1-0twqb553' failed to start.

It happens with both the above busybox as well as with a full ubuntu OS container, so I don't think that's relevant.

Unfortunately lxc-start-ephemeral has no --logfile or --logpriority or similar debugging tools, and even with -k it doesn't keep a container or logs around for inspection.

dmesg might have something worth noting, look at the third line:

[ 465.694541] device vethQLD78J entered promiscuous mode
[ 465.694611] IPv6: ADDRCONF(NETDEV_UP): vethQLD78J: link is not ready
[ 465.726758] overlayfs: missing upperdir or lowerdir or workdir
[ 465.744501] eth0: renamed from vethWRSY7B
[ 465.745673] IPv6: ADDRCONF(NETDEV_CHANGE): vethQLD78J: link becomes ready
[ 465.745693] lxcbr0: port 1(vethQLD78J) entered forwarding state
[ 465.745697] lxcbr0: port 1(vethQLD78J) entered forwarding state
[ 465.746618] lxcbr0: port 1(vethQLD78J) entered disabled state
[ 465.747134] device vethQLD78J left promiscuous mode
[ 465.747140] lxcbr0: port 1(vethQLD78J) entered disabled state

So maybe overlayfs changed its semantics somehow in the new kernel?

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: lxc 1.1.0~alpha3-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-28.38-generic 3.16.7-ckt1
Uname: Linux 3.16.0-28-generic x86_64
ApportVersion: 2.15.1-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Jan 11 12:45:06 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-11-20 (52 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Alpha amd64 (20141119)
SourcePackage: lxc
UpgradeStatus: No upgrade log present (probably fresh install)
defaults.conf:
 lxc.network.type = veth
 lxc.network.link = lxcbr0
 lxc.network.flags = up
 lxc.network.hwaddr = 00:16:3e:xx:xx:xx
lxc.conf: lxc.lxcpath = /srv/lxc
modified.conffile..etc.apparmor.d.abstractions.lxc.start.container: [modified]
mtime.conffile..etc.apparmor.d.abstractions.lxc.start.container: 2015-01-08T13:09:02.545927