BookStack/app/Http/Middleware/CheckForMaintenanceMode.php

<?php

namespace BookStack\Http\Middleware;

use Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode as Middleware;

class CheckForMaintenanceMode extends Middleware
{
    /**
     * The URIs that should be reachable while maintenance mode is enabled.
     *
     * @var array
     */
    protected $except = [
        //
    ];
}