HTTP Origin header (http://app.example.local:8080) didn't match request.base_url (http://app.example.local)

I am creating an app which has two parts:

• Backend (with Rails 6)
• FrontEnd (with React)

So, locally I setup nginx server for proxying.

But when I try to do a POST request, I got this error:

HTTP Origin header (http://app.example.local:8080) didn't match request.base_url (http://app.example.local)

The nginx host (app.example.local) has the following configuration:

server {
  listen 8080;
  listen [::]:8080;

  server_name app.example.local;

  access_log /usr/local/var/log/nginx/app.example.local_access.log;
  error_log /usr/local/var/log/nginx/app.example.local_error.log;

  location / {
    proxy_set_header Host $host;
    #proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Real-IP $remote_addr;
    #proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    #proxy_set_header X-Forwarded-Port $server_port;
    proxy_set_header X-Forwarded-Host $host;

    proxy_pass http://127.0.1:3001;

    proxy_redirect off;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
  }

  location /api {
    proxy_pass http://app.example.local:3000;

    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-Port $server_port;
    proxy_set_header X-Forwarded-Host $host;

    proxy_redirect off;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
  }
}

Making a GET request to http://app.example.com:8080/api/posts, for example, works fine. But when I try a POST request to http://app.example.com:8080/api/posts I am getting the following error:

Started POST "/api/posts" for 127.0.0.1 at 2020-07-29 15:20:11 +0100
Processing by API::PostsController#create as JSON
  Parameters: {"post"=>{"title"=>"Demo 2", "content"=>"Post Demo 2"}}
HTTP Origin header (http://app.example.local:8080) didn't match request.base_url (http://app.example.local)
Completed 422 Unprocessable Entity in 0ms (ActiveRecord: 0.0ms | Allocations: 432)


  
ActionController::InvalidAuthenticityToken (ActionController::InvalidAuthenticityToken):
  
actionpack (6.0.3.2) lib/action_controller/metal/request_forgery_protection.rb:215:in `handle_unverified_request'
...

I setup backend CORs like this:

    allow do
      origins %r{https?://(.*?)\.example\.local:8080$}

      resource '*',
        headers: :any,
        methods: %i[get post put patch delete options head],
        credentials: true
    end

And I am sending correctly the CSRF_TOKEN.

I already saw similar error here but the solutions are not fixing my problem. Most of them the problem are being caused by SSL. But since I am not using SSL this should not be the cause.

What am I missing?