Issue with overlay networks and Zookeeper as cluster store

I set up zookeeper as cluster store, but for some reason, I can't create overlay networks:

$ cat /etc/docker/daemon.json
{
        "cluster-store": "zk://zookeeper.example.com/",
        "cluster-advertise": "eth1:2376",
        "debug": true
}


$ docker info 
...
Cluster Store: zk://zookeeper.example.com/
Cluster Advertise: <ip>:2376
...

$ docker network create -d overlay testnet
Error response from daemon: pool configuration failed because of zk: unknown error

root@db05:~# tailf /var/log/upstart/docker.log
time="2017-10-31T14:38:42.546865312Z" level=debug msg="Calling GET /_ping"
time="2017-10-31T14:38:42.547828124Z" level=debug msg="Calling POST /v1.32/networks/create"
time="2017-10-31T14:38:42.547962672Z" level=debug msg="form data: {\"Attachable\":false,\"CheckDuplicate\":true,\"ConfigFrom\":null,\"ConfigOnly\":false,\"Driver\":\"overlay\",\"EnableIPv6\":false,\"IPAM\":{\"Config\":[],\"Driver\":\"default\",\"Options\":{}},\"Ingress\":false,\"Internal\":false,\"Labels\":{},\"Name\":\"testnet\",\"Options\":{},\"Scope\":\"\"}"
time="2017-10-31T14:38:42.558957407Z" level=debug msg="failed to get networks for scope global: zk: node does not exist"
time="2017-10-31T14:38:42.559239286Z" level=debug msg="Allocating IPv4 pools for network testnet (e7721dc0ed40d6741f6a82fc020cb07f462f1f94afdc5d6ff07baa1a2bd8be47)"
time="2017-10-31T14:38:42.559270326Z" level=debug msg="RequestPool(GlobalDefault, , , map[], false)"
time="2017-10-31T14:38:42.576364230Z" level=debug msg="FIXME: Got an API for which error does not match any expected type!!!: pool configuration failed because of zk: unknown error" error_type=types.internal module=api
time="2017-10-31T14:38:42.576394624Z" level=error msg="Handler for POST /v1.32/networks/create returned error: pool configuration failed because of zk: unknown error"
time="2017-10-31T14:38:42.576418525Z" level=debug msg="FIXME: Got an API for which error does not match any expected type!!!: pool configuration failed because of zk: unknown error" error_type=types.internal module=api

Looks like /docker/network path is missing on Zookeeper:

[zk: localhost:2181(CONNECTED) 9] ls /docker
[nodes]
[zk: localhost:2181(CONNECTED) 10] ls /docker/nodes
[<ip>:2376]

Is there something I can do here? Can I add manually some nodes to zookeeper?

Start by reproducing docker network create -d overlay testnet with the shown daemon.json and inspect the daemon log for the ZooKeeper path errors. Check the /docker and /docker/nodes entries in ZooKeeper and trace the global network lookup; done means identifying the cause of the missing path or documenting the required initialization steps.