本番環境で「419 Page Expired」

前提

herokuにデプロイしたlaravelのアプリが、
419エラーになります。

formで送信時にエラーが発生しています。

該当のソースコード

php
1        <form method="post" action="{{ asset('user/add') }}" enctype="multipart/form-data">
2            @csrf
3:
4:
5            <div class="flex_btn margin_top">
6                <input class="btn btn-outline-info" type="button" onclick="history.back()" value="戻る">
7                <input class="btn btn-outline-dark submit_btn" type="submit" value="次へ">
8            </div>
9        </form>

試したこと

・@csrfの有無確認
・app/Providers/AppServiceProvider.phpに下記追記

AppServiceProvider.php
1namespace App\Providers;
2
3use Illuminate\Support\ServiceProvider;
4use Illuminate\Support\Facades\App;
5use Illuminate\Support\Facades\URL;
6
7class AppServiceProvider extends ServiceProvider
8{
9    /**
10     * Register any application services.
11     *
12     * @return void
13     */
14    public function register()
15    {
16        //
17    }
18
19    /**
20     * Bootstrap any application services.
21     *
22     * @return void
23     */
24    public function boot()
25    {
26        if (App::environment(['production'])) { //追記
27            URL::forceScheme('https');//追記
28        }//追記
29    }
30}

・TrustProxies.phpに追記

TrustProxies.php
1namespace App\Http\Middleware;
2
3use Fideloper\Proxy\TrustProxies as Middleware;
4use Illuminate\Http\Request;
5
6class TrustProxies extends Middleware
7{
8    /**
9     * The trusted proxies for this application.
10     *
11     * @var array|string
12     */
13    protected $proxies = '*'; //追記
14
15    /**
16     * The headers that should be used to detect proxies.
17     *
18     * @var int
19     */
20    protected $headers = Request::HEADER_X_FORWARDED_ALL;
21}