🇨🇿Czech Republic @XSramik

Hello Cilefen, thank you for looking into it so quickly. Despite the log message, the problem was within another module which implements CSV serialization in (likely) outdated way. I just replaced that module and everything works well.

I am closing this as the 'rest.module' works as should.

XSramik → created an issue.

It turns out that the best way to implement this is to use $http_client_factory. Here's a working example in case anyone finds it useful.

<?php
namespace Drupal\module\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Site\Settings;
use Drupal\Core\Http\ClientFactory;

class DevForm extends FormBase {
  protected $httpClient;
  protected $cookieJar;
  protected $aquaponi;

  public function __construct(ClientFactory $http_client_factory) {
    $this->cookieJar = new \GuzzleHttp\Cookie\CookieJar;
    $this->company= (object)(Settings::get('default','company')['company']);
    $this->httpClient = $http_client_factory->fromOptions([
      'base_uri' => $this->company->base_uri,
      'cookies'  => $this->cookieJar,
    ]);

    // Login to the COMPANY API (get the token)
    if(empty($this->cookieJar->getCookieByName($this->aquaponi->cookie)))
    {
      $request = $this->httpClient->request('POST', 'login', [
        'json' => [
          'username' => $this->company->username,
          'password' => $this->company->password
        ]
      ]);
    }
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('http_client_factory')
    );
  }

...

The authorization token and 'base_uri' are then used by default with the following call:

$this->httpClient->request('GET', 'data');

Source: https://drupalize.me/blog/speak-http-drupalhttpclient

I have installed 2.0.5. Works, thank you.

XSramik → created an issue.