Nginx反向代理未将某些页面传递给app server

我有一个NGINX服务器作为OTRS应用服务器的反向代理运行

upstream otrs_server {
        server X.X.X.X;
}


server {
        listen 17031;

        access_log /var/log/nginx/otrs.access.log;

        location / {
                include  /etc/nginx/mime.types;
                default_type  application/octet-stream;
                sendfile        on;
                keepalive_timeout  65;
                gzip  on;
                proxy_set_header Host $http_host;
                proxy_pass http://otrs_server;
         }

}

我可以看到OTRS应用服务器的登录页面，当我检查

/var/log/nginx/OTRS.access.log

 [26/Apr/2016:11:47:56 +0200] "GET /otrs/index.pl HTTP/1.1" 200 3951 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.112 Safari/537.36"

2016/04/26 11:47:56 [error] 2353#2353: *279897 open() "/etc/nginx/html/otrs-web/skins/Agent/KIX4OTRS/css-cache/img/logo.png" failed (2: No such file or directory), request: "GET /otrs-web/skins/Agent/KIX4OTRS/css-cache/img/logo.png HTTP/1.1", referrer: "http://X.X.X.X:17031/otrs/index.pl"

但是所有的CSS和徽标都丢失了，我还可以看到

/var/log/nginx/error.log中的错误

 [26/Apr/2016:11:47:56 +0200] "GET /otrs/index.pl HTTP/1.1" 200 3951 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.112 Safari/537.36"

2016/04/26 11:47:56 [error] 2353#2353: *279897 open() "/etc/nginx/html/otrs-web/skins/Agent/KIX4OTRS/css-cache/img/logo.png" failed (2: No such file or directory), request: "GET /otrs-web/skins/Agent/KIX4OTRS/css-cache/img/logo.png HTTP/1.1", referrer: "http://X.X.X.X:17031/otrs/index.pl"

对于一些静态页面，nginx使用/etc/nginx/html/而不是

有人知道为什么会发生这种情况吗？
我猜OTRS应用程序服务器不知道它正在端口17031后面代理，并且正在生成引用默认端口的内容。