Firestore with PHP - Google\Cloud\Core\Exception\[DeadlineExceededException|ServiceException]

Question

I'm running a PHP project with theses dependencies :

{
    "require": {
        "slim/slim": "^3.9.2",
        "php": "^8.0.0",
        "slim/psr7": "^1.5",
        "google/cloud-firestore": "^1.20",
        "grpc/grpc": "^1.42",
        "google/cloud": "^0.175.0"
    }
}

I wan't to fetch data from a firestore database.

my php code :

<?php
session_start();

require_once __DIR__ . DIRECTORY_SEPARATOR . 'vendor' . DIRECTORY_SEPARATOR . 'autoload.php';

$conf = [
    "keyFilePath" => __DIR__ . DIRECTORY_SEPARATOR . 'src' . DIRECTORY_SEPARATOR . 'conf' . DIRECTORY_SEPARATOR . 'myfile.json'
];

use Google\Cloud\Firestore\FirestoreClient;

$db = new FirestoreClient($conf);
$user = $db->collection("users")->where('email', '=', "an-email")->documents();

I've tried many tutorials and the official doc but i've theses errors randomely. In many cases (80%) :

Type: Google\Cloud\Core\Exception\ServiceException
Code: 14
Message: { "message": "failed to connect to all addresses", "code": 14, "status": "UNAVAILABLE", "details": [] }
File: /users/home/arnoux23u/www/charlemiapp/charlemiapp-auth/vendor/google/cloud/Core/src/GrpcRequestWrapper.php
Line: 257

and in some other cases :

Type: Google\Cloud\Core\Exception\DeadlineExceededException
Code: 4
Message: { "message": "Deadline Exceeded", "code": 4, "status": "DEADLINE_EXCEEDED", "details": [] }
File: /users/home/arnoux23u/www/charlemiapp/charlemiapp-auth/vendor/google/cloud/Core/src/GrpcRequestWrapper.php
Line: 257

I don't know what's the problem. The app is running fine on NodeJS.

I precise that I'm not the owner of the machine which host the app. The webmaster has installed grpc extension (1.43.0).