This connection is not secure. Logins entered here could be compromised

This is on Geoserver which uses Jetty/Java. I followed this article on a Debian machine and then added SSL with Certbot (my padlock is fine). The message comes up when I click the user/password input box. It seems like a mixed content warning but I don't know what Nginx or Geoserver configuration I need to change. I have v2.22.2.

server {

    server_name mydomain.com www.mydomain.com;

    location / {
        proxy_pass http://127.0.0.1:8080;
        include proxy_params;
        proxy_set_header X-Forwarded-Host $host:$server_port;
        proxy_set_header X-Forwarded-Server $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/mydomain.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/mydomain.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
    add_header 'Content-Security-Policy' 'upgrade-insecure-requests';
}
server {
    if ($host = mydomain.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    listen 80;
    listen [::]:80;

    server_name mydomain.com www.mydomain.com;
    return 404; # managed by Certbot

    location / {
        # root /path/to/myapp/public;
        proxy_set_header X-Forwarded-Host $host:$server_port;
        proxy_set_header X-Forwarded-Server $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://172.0.0.1:8080;
    }
    add_header 'Content-Security-Policy' 'upgrade-insecure-requests';
}