It makes it sound like either the cookie auth or x-cloud-authorization is missing from the connection.
the error seen isn't coming from vCloud, but probably the web server stating that the authorization is missing.
you can check the rejection for the API call in the request log. Log name format is YYYY_MM_DD then request then maybe some digits then .log.
in there there will be the URL and the HTTP status code. you should be able to grep it for the HTTP status code of 401.
The only exception to this is if you have an API gateway/security/monitoring tool between Electric Commander and vCloud Director's address.