Upgraded to Laravel 5.6

app/Http/Middleware/CheckForMaintenanceMode.php

@@ -0,0 +1,17 @@
+<?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 = [
+        //
+    ];
+}