Server was unable to process request. ---> Index was out of range - php

Question

I'm getting this error:
Fatal error: Uncaught SoapFault exception: [soap:Server] Server was unable to process request. ---> Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index in /var/www/index.php:30 Stack trace: #0 /var/www/index.php(30): SoapClient->__call() #1 {main} thrown in /var/www/index.php on line 30
My php-code is:
<?php
ini_set('error_log', '/var/www/php.log');
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
date_default_timezone_set('UTC');
include("include/connection.php");
$client = new SoapClient("https:// (LINK TO API) /ExternalApiWebService.asmx?wsdl", array('trace' => 1));
// **--** Login - START **--**
$LoginResult = $client->Login(
array("userName" => "USERNAME", "password" => "PASSWORD"
)
);
print "<pre>";
print_r ($LoginResult);
print "</pre>";
$SessionId = $LoginResult->LoginResult;
// **--** Login - END **--**
// **--** GetTriplogUnitPositions - START **--**
$GetTriplogUnitPositionsResult = $client->GetTriplogUnitPositions(
array("sessionId" => "".$SessionId.""
)
);
print "<pre>";
print_r ($GetTriplogUnitPositionsResult);
print "</pre>";
// **--** GetTriplogUnitPositions - END **--**
?>
Can anyone help me or give me a hint of whats wrong?