vagrant box fails with private network interface

Hello,

I am attempting to use the Xenial vagrant box announced here:
https://lists.ubuntu.com/archives/ubuntu-devel/2016-January/039070.html

The default setup, as described in the announcement, works fine.
However, it fails if I add a private network interface in Vagrantfile:

    config.vm.network "private_network", ip: "192.168.50.100"

Output of "vagrant up":

deepstar@perdix:/tmp/example$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Importing base box 'http://cloud-images.ubuntu.com/xenial/current/xenial-server-cloudimg-amd64-vagrant.box'...
==> default: Matching MAC address for NAT networking...
==> default: Setting the name of the VM: example_default_1453900316714_21930
==> default: Fixed port collision for 22 => 2222. Now on port 2201.
==> default: Clearing any previously set network interfaces...
==> default: Preparing network interfaces based on configuration...
    default: Adapter 1: nat
    default: Adapter 2: hostonly
==> default: Forwarding ports...
    default: 22 (guest) => 2201 (host) (adapter 1)
==> default: Booting VM...
==> default: Waiting for machine to boot. This may take a few minutes...
    default: SSH address: 127.0.0.1:2201
    default: SSH username: ubuntu
    default: SSH auth method: password
    default: Warning: Remote connection disconnect. Retrying...
    default: Warning: Authentication failure. Retrying...
    default:
    default: Inserting generated public key within guest...
    default: Removing insecure key from the guest if it's present...
    default: Key inserted! Disconnecting and reconnecting using new SSH key...
==> default: Machine booted and ready!
==> default: Checking for guest additions in VM...
==> default: Configuring and enabling network interfaces...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

/sbin/ifdown eth1 2> /dev/null

Stdout from the command:

Stderr from the command:

sudo: unable to resolve host ubuntu-xenial
mesg: ttyname failed: Inappropriate ioctl for device

The version of the vagrant box is 20160126.1
I am using Vagrant 1.8.1 on OSX 10.11.2 if that matters.
Other vagrant boxes I've tested work fine with this extra network interface in the Vagrantfile.

kr,
-- Steven