Getting HTTP 502 bad gateway error on .net core API with docker and Nginx

I have two docker container of my 2 different net core API project running on my machine (Linux) on respective ports 3333:80 and 6666:8088. I have deployed their front end part on Nginx server each having its own configuration in sites-available folder. The problem is that my 1st container (API) is working fine, getting response from front end application as well as from Postman but the 2nd container is not working, throwing this error HTTP 502 bad gateway and error msg:

recv() failed (104: Connection reset by peer) while reading response header from upstream

What's wrong over here? Kindly help me to resolve this issue. Following are my config files: nginx.conf:

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
    worker_connections 768;
    # multi_accept on;
}

http {

    ##
    # Basic Settings
    ##
    client_max_body_size 50M;   
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    # server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

site1.conf

server {
    listen 80 default_server;

    server_name _;

    root /var/www/app.admin-crm.com;
    index index.html;

    location /api/ {
        proxy_pass      http://127.0.0.1:3333/api/;
        proxy_redirect  off;
        proxy_set_header        X-Real-IP $remote_addr;
        proxy_set_header        X-Forwarded_For $proxy_add_x_forwarded_for;
        proxy_set_header        X-Forwarded_Host $server_name;
        add_header      Access-Control_Allow-Credentials true;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        client_max_body_size    50M;
    }

    location / {
        try_files $uri $uri/ /index.html;
    }
 }

site2.conf:

server {
    listen 8088 default_server;

    server_name _;

    root /var/www/stilaar-web;
    index index.html;

    location /api/ {
        proxy_pass      http://127.0.0.1:6666/api/;
        proxy_redirect  off;
        proxy_set_header        X-Real-IP $remote_addr;
        proxy_set_header        X-Forwarded_For $proxy_add_x_forwarded_for;
        proxy_set_header        X-Forwarded_Host $server_name;
        add_header      Access-Control_Allow-Credentials true;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $host;
        client_max_body_size    50M;
    }

    location / {
        try_files $uri $uri/ /index.html;
    }

}