Error code 511 - A required query string argument is missing

I am trying to update user custom fields using the community V1 REST API, and I am getting a server error code - 511 Network Authentication Required - when doing a POSt operation over the user. The object than contains the custom fields is "settings" so I'm trying the following:

------------------------------------------------------------------------

POST /restapi/v1/users/id/6463/settings/name/custom.field.fiel_name/set?custom.field.fiel_name=SomeValue&restapi.response_format=json&restapi.session_key={sOmEsEsSiOn.}

HTTP/1.1
Authorization: Basic someAuth
Host: community.staging.company.com
Content-Length: 0

------------------------------------------------------------------------

and getting the response:

{
    "response": {
    "status": "error",
       "error": {
          "code": 511,
          "message": "A required query string argument is missing."
       }
    }
}

After doing a self request, I get the full list of available fields to update.

/restapi/v1/users/self/settings

{
    "response": {
    "status": "success",
    "settings": {
    "type": "settings",
    "href": "\/users\/id\/6463\/settings",
    "setting": [

       ...

    ]

]

Any ideas?

Thanks,

Ivan.