HTTP redirection to HTTPS not working properly

I have set up Nginx as a reverse proxy as well as using SSL.

My website works on https://example.com as well as https://www.example.com.

But when I open a private browser tab and search for http://example.com it loads for ever. My nginx config looks as following (example.com is of course for this purpose):

map $sent_http_content_type $expires {
    "text/html"                 epoch;
    "text/html; charset=utf-8"  epoch;
    default                     off;
}


server {

        server_name example.com www.example.com;
        root /var/www/html;

	# G-Zip config
        gzip off;

        # Entry point for all requests (redirect to frontend)
        location / {
                expires $expires;

                proxy_redirect off;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection 'upgrade';
                proxy_cache_bypass $http_upgrade;
                proxy_set_header Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_set_header X-Forwarded-Proto $scheme;
                proxy_read_timeout 1m;
                proxy_connect_timeout 1m;
                proxy_pass http://localhost:3000;
        }

        # Redirect to API-Endpoint of the backend
        location /api {
                proxy_pass http://localhost:8080/api;
        }

        # Redirect to public/images endpoint of the backend
        # The frontend path is always /img for images
        location /img {
                proxy_pass http://localhost:8080/images;
        }

    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot


}


server {

    if ($host = www.example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    if ($host = example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    listen 80 default_server;

    server_name example.com www.example.com;
    return 404; # managed by Certbot
}