Comment 2 for bug 1449633

Same problem here with Juju 2.0.2-yakkety-amd64

First controller created using:
juju bootstrap ovh juju-controller --metadata-source ~/simplestreams --bootstrap-series yakkety

On an Openstack cloud provider (OVH)

juju status
Machine State DNS Inst id Series AZ
0 started xx.xx.xx.xxx e3ce647b-0458-436f-a0cc-0df6b037972c yakkety nova
1 down pending yakkety
2 down pending yakkety
3 down pending yakkety
4 down pending yakkety

The problem raised after using
juju enable-ha
maintaining machines: 0
adding machines: 1, 2

No error shown (made 2 times without option, that is why I have machines 3 and 4).

I am completely stuck, no way to remove old down machines:
juju remove-machine 4 --force
ERROR no machines were destroyed: machine is required by the model

Is anything more needed for a reproductible scenario?