Error running on subdirectory with Nginx

Community Support
1

So I’m using Ubuntu 16 and I’ve installed rocket.chat with snap. I’ve setup systemd and SSL with Let’s Encrypt. Following is my Nginx config file and I keep getting 502 Bad Gateway when I go to app.andrew.at.tw/chat. Who can help me out?

# Upstreams
upstream backend {
    server 127.0.0.1:3000;
}

#HTTPS Server
server {

        root /var/www/app;

        # Add index.php to the list if you are using PHP
        index index.html index.htm index.nginx-debian.html;

        server_name app.andrew.at.tw;

        location ~/chat(.*)$ {
                proxy_pass http://backend/$1;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection "upgrade";
                proxy_set_header Host $http_host;

                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forward-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forward-Proto http;
                proxy_set_header X-Nginx-Proxy true;

                proxy_redirect off;
        }

    listen [::]:443 ssl; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/app.andrew.at.tw/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/app.andrew.at.tw/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}