Sending form data with guzzle throws server error

Question

I'm sending form data, "packed" in URL using Guzzle to my JasperReports which is installed on another server. URL consists of form data and it's super long.

All the time, I'm receiving 500 Internal Server Error.

 $headers = [
           'url' => $url,
       ];

 this->client = new Client(['base_uri' => 'http://localhost:8080/jasper-r/report.jsp?id=0']);

       try {
           $promise = $this->client->request('POST', ["headers" => $headers]);
           return response($promise->message_id);
       } catch (RequestException $e) {
           return abort($e->getMessage());