我有一个在 Debian 上运行 Plesk 12 的生产 Web 服务器。一个网站运行的是 Laravel 4.2，但我认为该错误不是 Laravel 的问题。我认为是 Apache/Nginx 的问题（Plesk 有 nginx 来缓存静态文件）。
该错误显示一个空白页，并显示消息“缓存失败”，这是完全随机的，我们无法重现该错误，因为我们有时会执行相同的操作，可能不会发生，有时会发生。
日志
[Wed Jul 15 04:56:19 2015] [error] [client 1] File does not exist: /var/www/vhosts/web_host.com/web_users
[Wed Jul 15 04:56:31 2015] [error] [client 1] File does not exist: /var/www/vhosts/web_host.com/web_users
[Wed Jul 15 04:56:41 2015] [error] [client 1] File does not exist: /var/www/vhosts/web_host.com/web_users
[Wed Jul 15 08:28:43 2015] [error] [client 2] Directory index forbidden by Options directive: /var/www/vhosts/web_host.com/public/uploads/posts/, referer: http://www.web_host.com/admin/posts
[Wed Jul 15 08:28:47 2015] [error] [client 2] Directory index forbidden by Options directive: /var/www/vhosts/web_host.com/public/uploads/posts/, referer: http://www.web_host.com/admin/posts
[Wed Jul 15 10:46:53 2015] [error] [client 3] File does not exist: /var/www/vhosts/web_host.com/web_users
[Fri Jul 17 08:33:03 2015] [error] [client 4] Directory index forbidden by Options directive: /var/www/vhosts/web_host.com/public/tmp/posts/files/
我在 Google 和本网站搜索，但一无所获，有谁能帮我解决这个问题？我该怎么办？我该从哪里开始搜索？