Why the strapi admin panel may not open?

Question

The Strap admin panel does not open. But the nextjs application received data from strapi. Nginx seems to be configured correctly.

connetion config/server.js

I don't see any errors in nginx logs

nginx sites-available strapi config

server {
    listen 80;
    server_name api.exemple.com;
    root /var/www/promo/nextjs-coupon/couponCMS;
    index index.js;

    location / {
        proxy_pass http://localhost:1337;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }
}

I changed the nginx config, but it didn't help