PHP - Server wa unable to process request ---> Object reference not set to an instance of an object

$ws_url   = "https://xxxxxxxxx.asmx?wsdl";

    $location = "https://xxxxx.asmx";

    define("TIMEOUT", 25);

    ini_set('default_socket_timeout', TIMEOUT);

   $opts = array( 'ssl'   => array( 'verify_peer'  => false));

   $streamContext = stream_context_create($opts);

    try{

        $client = new SoapClient($ws_url, array ("soap_version" => SOAP_1_1,"location" => $location,'keep_alive' => true,'trace' => 1,'cache_wsdl' => WSDL_CACHE_NONE,"stream_context" => $streamContext));

        $m = "AuntenticarCliente";

        $d = array("Usuario" => "user","Password" => "clave");

        try{

            $result = $client->__soapCall($m, $d);

            var_dump($result);

        } catch (Exception $ex) {

            var_dump($ex);

        }

    }catch (Exception $ex) {

        var_dump($ex);

    }

try{

	$client = new SoapClient($ws_url, array ("soap_version" => SOAP_1_1,"location" => $location,'keep_alive' => true,'trace' => 1,'cache_wsdl' => WSDL_CACHE_NONE,"stream_context" => $streamContext));

}catch (Exception $ex) {

	var_dump($ex);

}

$m = "AuntenticarCliente";

$d = array("Usuario" => "user","Password" => "clave");

try{

	$result = $client->__soapCall($m, $d);

	var_dump($result);

}catch(SoapFault $e) {

	var_dump($e);

}

<s:schema elementFormDefault="qualified" targetNamespace="http://xxxxxx.com/">

      <s:element name="AuntenticarCliente">

        <s:complexType>

          <s:sequence>

            <s:element minOccurs="0" maxOccurs="1" name="Usuario" type="s:string" />

            <s:element minOccurs="0" maxOccurs="1" name="Password" type="s:string" />

          </s:sequence>

        </s:complexType>

      </s:element>

      <s:element name="AuntenticarClienteResponse">

        <s:complexType>

          <s:sequence>

            <s:element minOccurs="0" maxOccurs="1" name="AuntenticarClienteResult" type="s:string" />

          </s:sequence>

        </s:complexType>

      </s:element>

object(SoapFault)#2 (10) {

  ["message":protected]=>

  string(96) "Server was unable to process request. ---> Object reference not set to an instance of an object."

  ["string":"Exception":private]=>

  string(0) ""

  ["code":protected]=>

  int(0)

  ["file":protected]=>

  string(33) "/home/proyecto/ws/core/net/test.php"

  ["line":protected]=>

  int(29)

  ["trace":"Exception":private]=>

  array(1) {

    [0]=>

    array(6) {

      ["file"]=>

      string(33) "/home/proyecto/ws/core/net/test.php"

      ["line"]=>

      int(29)

      ["function"]=>

      string(10) "__soapCall"

      ["class"]=>

      string(10) "SoapClient"

      ["type"]=>

      string(2) "->"

      ["args"]=>

      array(2) {

        [0]=>

        string(18) "AuntenticarCliente"

        [1]=>

        array(2) {

          ["Usuario"]=>

          string(4) "user"

          ["Password"]=>

          string(5) "clave"

        }

      }

    }

  }

  ["previous":"Exception":private]=>

  NULL

  ["faultstring"]=>

  string(96) "Server was unable to process request. ---> Object reference not set to an instance of an object."

  ["faultcode"]=>

  string(11) "soap:Server"

  ["detail"]=>

  string(0) ""

}