Nginx gives 502 Bad Gateway when i try to acess my node js aplication running on different docker container

So I’m trying to use NGINX as a reverse proxy for 2 react apps and 1 node js api. Each in separate docker containers.

The restapi service must be hosted on a seperated port as the portal by default. The nginx proxy configuration must include a route for the Portal on port 443 and a route for the restapi and admin portal must have a seperate route entry on a different Port like 8443.

With my current nginx configuration I expect the following behavior: localhost leads to one react app called portal, localhost/adminportal/ -> leads to another react app admin portal, localhost/api-portal/ -> leads to the api fo the portal app, and localhost/api-adminportal/ -> leads to the api fo the admin portal app

But at this moment only the first and second example (localhost/ and localhost/adminportal/) is working properly, the other two show a nginx error page.

And When i try to acess localhost/api-portal/ i have this error in my log file of my nginx container:

2022/04/10 20:43:33 [error] 30#30: *57 upstream prematurely closed connection while reading response header from upstream, client: 172.28.0.1, server: localhost, request: "GET /api/ HTTP/1.1", upstream: "http://172.28.0.3:3333/", host: "localhost:8443"

172.28.0.1 - - [10/Apr/2022:20:43:33 +0000] "GET /api/ HTTP/1.1" 502 497 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.75 Safari/537.36" "-"

This is my nginx configuration:

server {
    listen 80;
    server_name localhost 127.0.0.1 0.0.0.0;
    return 301 https://$server_name$request_uri;
}
    
server {
    listen 443 ssl;
    server_name localhost 127.0.0.1 0.0.0.0;

    ssl_certificate /etc/nginx/cert/cert.pem;
    ssl_certificate_key /etc/nginx/cert/key.pem;

    location / {
        proxy_pass http://fpr-frontend:3000/;
    }


    # include /etc/nginx/conf.d/koma/*.conf;

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }port
}

server {
    listen 8443 ssl;
    server_name localhost 127.0.0.1 0.0.0.0;

    ssl_certificate /etc/nginx/cert/cert.pem;
    ssl_certificate_key /etc/nginx/cert/key.pem;

     location /adminportal/ {
        proxy_pass http://fpr-adminportal:3001/adminportal/;
    }

    location /api-adminportal/ {
        proxy_pass http://fpr-backend:3335/;
    }

    location /api-portal/ {
        proxy_pass http://fpr-backend:3333/;
    }

    # include /etc/nginx/conf.d/koma/*.conf;

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }

}

Below is my docker-compose file:

version: "3.2"

volumes:
  mongodata:

services:
  fpr-backend:
    build:
      context: .
      dockerfile: Dockerfile.backend
    image: fpr-backend
    env_file: ./config/fpr-backend.env
    environment:
      FRONTEND_URL: http://localhost:8000/
      FRONTEND_ADMIN_PORTAL_URL: https://localhost:8443/adminportal/
    
    depends_on:
      - mongo
    expose:
      - "3333"
      - "3335"
    ports:
      - "3333:3333/tcp"
      - "3335:3335/tcp"
    command: ["yarn", "start"]

  fpr-frontend:
    build:
      context: .
      dockerfile: Dockerfile.frontend
      args:
       VITE_BACKEND_URL: https://localhost:3333/
    image: fpr-frontend
    env_file: ./config/fpr-frontend.env
    depends_on:
      - fpr-backend
    expose:
      - "3000"
    command: ["yarn", "serve", "--host=fpr-frontend", "--port=3000"]

  fpr-adminportal:

    stdin_open: true # docker run -i
    tty: true        # docker run -t
    build:
      context: .
      dockerfile: Dockerfile.adminportal
      args:
        PORT: 3001
        REACT_APP_API_URL:  https://localhost:3335/
  
    image: fpr-adminportal
    env_file: ./config/fpr-adminportal.env
    environment: 
      - PUBLIC_URL=https://localhost:8443/adminportal/
    depends_on:
      - fpr-backend
    expose:
      - "3001"
    command: ["yarn","dev"]  

  mongo:
    image: mongo
    env_file: ./config/mongo.env
    volumes:
      - mongodata:/etc/mongo


  nginx:
    image: nginx
    depends_on:
      - fpr-backend
      - fpr-frontend
      - fpr-adminportal
    volumes:
      - ./config/nginx/conf.d/:/etc/nginx/conf.d/
      - ./modules/fpr-backend/certificates/UserPortal:/etc/nginx/cert
    expose:
      - "443"
      - "8443"
    ports:
      - "8000:80/tcp"
      - "443:443/tcp"
      - "8443:8443/tcp"

fpr-backend(api) dockerfile:

FROM node:14-alpine
WORKDIR /source
COPY ./modules/fpr-backend/package.json .
COPY ./modules/fpr-backend/yarn.lock .
RUN yarn
COPY ./modules/fpr-backend/ .
EXPOSE 3333
EXPOSE 3335
CMD [ "yarn" , "start" ]