TokenMismatchException, but where is it coming from?

I've got a published website, since yesterday it did look to be fine.
Yerstrday and today instead, I've got an error into the log file.
In both cases the error is

[2017-02-25 11:23:26] production.ERROR: exception 'Illuminate\Session\TokenMismatchException' in 
/phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Foundation/Http/Middleware/VerifyCsrfToken.php:67
Stack trace:
#0 [internal function]: Illuminate\Foundation\Http\Middleware\VerifyCsrfToken->handle(Object(Illuminate\Http\Request), Object(Closure))
#1 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(136): call_user_func_array(Array, Array)
#2 [internal function]: Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}(Object(Illuminate\Http\Request))
#3 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Routing/Pipeline.php(32): call_user_func(Object(Closure), Object(Illuminate\Http\Request))
#4 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Session/Middleware/StartSession.php(64): Illuminate\Routing\Pipeline->Illuminate\Routing\{closure}(Object(Illuminate\Http\Request))
#5 [internal function]: Illuminate\Session\Middleware\StartSession->handle(Object(Illuminate\Http\Request), Object(Closure))
#6 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(136): call_user_func_array(Array, Array)
#7 [internal function]: Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}(Object(Illuminate\Http\Request))
#8 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Routing/Pipeline.php(32): call_user_func(Object(Closure), Object(Illuminate\Http\Request))
#9 [internal function]: Illuminate\Routing\Pipeline->Illuminate\Routing\{closure}(Object(Illuminate\Http\Request))
#10 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php(103): call_user_func(Object(Closure), Object(Illuminate\Http\Request))
#11 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php(132): Illuminate\Pipeline\Pipeline->then(Object(Closure))
#12 /phoing.com/public_html/vendor/laravel/framework/src/Illuminate/Foundation/Http/Kernel.php(99): Illuminate\Foundation\Http\Kernel->sendRequestThroughRouter(Object(Illuminate\Http\Request))
#13 /phoing.com/public_html/index.php(54): Illuminate\Foundation\Http\Kernel->handle(Object(Illuminate\Http\Request))
#14 {main}

I can't understand which form is generating this problem, I'm not even sure if it is because of a user or a bot.
This is how I handle the exceptions

public function render($request, Exception $e)
{
    switch($e)
    {
        // handles the token expiration, i.e. when a user waits too much before to submit a form
        case ($e instanceof TokenMismatchException):
            $title = trans('error.token_expired_title');
            $message = trans('error.token_expired');
            return response()->view('errors.custom', compact('title','message'));
            break;

        case ($e instanceof HttpException):
        case ($e instanceof NotFoundHttpException):
        case ($e instanceof HttpResponseException):
        case ($e instanceof AuthenticationException):
        case ($e instanceof AuthorizationException):
        case ($e instanceof ValidationException && $e->getResponse()):
            return parent::render($request, $e);
            break;

        default:
            $title = trans('error.unexpected_title');
            $message = trans('error.unexpected');
            return response()->view('errors.custom',compact('title','message'));
            break;
    }
}

What do you think? What should I check?

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