Magento 2.4.3 – Admin Dashboard Page Not Loading After Login

I faced the same problem with a Magento setup with Nginx.

I share you my working configuration which solve this CORS problem for an headless Magento use case (API).

By defaut Magento do not accept OPTIONS call with API. So a workaround is to return a 200 response for all OPTIONS call on API :

location /rest {
    if ($request_method = 'OPTIONS') {
        add_header 'Access-Control-Allow-Origin' '*';
        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS';
        add_header 'Access-Control-Allow-Headers' 'DNT,Authorization,X-CustomHeader,Keep-Alive,User-Agent,Origin,Referer,X-HTTP-Method-Override,X-Accept-Charset,X-Accept,Accept,Access-Control-Request-Method,Access-Control-Request-Headers,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type';
        add_header 'Access-Control-Max-Age' 1728000;
        add_header 'Content-Type' 'text/plain charset=UTF-8';
        add_header 'Content-Length' 0;
        return 200;
    }
}

Some Allow-Headers is not mandatory, try to test which are on your case.

Then add headers on all other requests :

location ~ (index|get|static|report|404|503)\.php$ {
    .....
    .....

    add_header 'Access-Control-Allow-Origin' '*' always;
    add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
    add_header 'Access-Control-Allow-Headers' 'DNT,Authorization,X-CustomHeader,Keep-Alive,User-Agent,Origin,Referer,X-HTTP-Method-Override,X-Accept-Charset,X-Accept,Accept,Access-Control-Request-Method,Access-Control-Request-Headers,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type' always;
}

Make sure to keep the always option in order to add these headers for response code other than : 200, 201, 204, 206, 301, 302, 303, 304, 307, or 308

Make sure you do not have basic auth authentication behind. If not, update the configuration in order to allow the OPTIONS CORS request made from your browser.

Nginx Not pass Access-Control-Allow-Origin

by using below module it will pass the header in API
It can update from Store >> Config >> General >> WEb Change * to any url you want to access

https://github.com/splashlab/magento-2-cors-requests