Due a bug in the Python SocketServer when the client disconnects suddently it will raise that exception (see http://bugs.python.org/issue14574).
New versions of Ironic switched to using oslo.service (https://review.openstack.org/#/c/219298/), so this isn't an issue any more.
The automatic restart of the service will fail.
Workaround (if any):
Manually restart the service with "systemctl restart openstack-ironic-api"