Uploading Fails using filestack-php

[not-original]

Installed with Composer

Using the following code, results in:
Errored uploading to s3503

require("../../../vendor/autoload.php");
use Filestack\FilestackClient;
use Filestack\Filelink;
use Filestack\FilestackException;

$filepath="../../../Docs/12.spring.clinic.assignments.xlsx";
$client = new FilestackClient('************');
try {
    $filelink = $client->upload($filepath);
    var_dump($filelink);
} catch (FilestackException $e) {
    echo $e->getMessage();
    echo $e->getCode();
}