rebalance_primaries fails with a 404 returncode

[mbstein]

We're new to clusters, so probably we made a stupid mistake or did not yet read an important chapter in the manual.
Trying to use the REST API on the clustermaster to redistribute primary buckets on a new cluster with version 6.0.1 we get a 404 response (the only one NOT in the manual).
What could be possibly do (in addition to sending anything through a loadbalacing forwarder and wait for a few month) ?

HTTP/1.1 404 Not Found
...
Server: Splunkd

<?xml version="1.0" encoding="UTF-8"?>
<response>
  <messages>
    <msg type="ERROR">In handler 'clustermastercontrol': Invalid action for this internal handler (handler: clustermastercontrol, supported: list|doc, wanted: edit).</msg>
  </messages>
</response>

[mbstein]

adding another "/control/" to the request helped a lot
If in doubt, cut & paste the complete URL