Authentication problem with Deploy module

I'm trying to push content from a staging to a production server, using the 7.x-2.0-alpha2 Deploy module. The servers have been set up according to the instructions, and the deployment plan and end point have been defined.

On the target server, I checked http://myhost/xxxx/services/rest/
The page displayed "Services Endpoint "xxxx" has been setup successfully." It also finds /xxx/services/rest/user/login

When I deploy new content, the operation fails. From the staging server log:

DeployAuthenticationException: Authentication error: 401 Unauthorized: Missing required argument name in DeployAuthenticatorSession->deploy() (line 54 of /var/www/html/yyyy/sites/all/modules/deploy/plugins/DeployAuthenticatorSession.inc).

This is the authentication request that is made:

Authentication request: http://myhost/xxxx/services/rest/user/login
Array
(
    [method] => POST
    [headers] => Array
        (
            [Content-Type] => application/json
        )

    [data] => {"username":"root","password":"(secret)"}
)

And here is the response:

Authentication response:
stdClass Object
(
    [request] => POST /xxxx/services/rest/user/login HTTP/1.0
Content-Type: application/json
User-Agent: Drupal (+http://drupal.org/)
Host: myhost
Content-Length: 41

{"username":"root","password":"(secret)"}
    [data] => 
    [protocol] => HTTP/1.0
    [status_message] => Unauthorized: Missing required argument name
    [headers] => Array
        (
            [date] => Tue, 13 Aug 2013 00:34:10 GMT
            [server] => Apache/2.2.3 (Red Hat)
            [x-powered-by] => PHP/5.3.27
            [expires] => Sun, 19 Nov 1978 05:00:00 GMT
            [last-modified] => Tue, 13 Aug 2013 00:34:10 +0000
            [cache-control] => no-cache, must-revalidate, post-check=0, pre-check=0
            [etag] => "1376354050"
            [content-length] => 0
            [connection] => close
            [content-type] => text/html; charset=UTF-8
        )

    [code] => 401
    [error] => Unauthorized: Missing required argument name
)

The log file on the production server isn't much help. I can see the request being made, but there are no error messages.

Any ideas? I can't believe the parameter names don't match, but it's a mystery as to what the receiving service is expecting.