Nginx reverse proxy breaks my website

I created a subdomain and changed its nginx config file to:

server {
    listen 80;
    server_name sub.domain.com;

    location / {
        proxy_pass http://127.0.0.1:8989;
    }
}

Everything works fine, the page loads except when I enter a login into my website the screen turns black. It only lets me login if I go to domain.com:8989

What do I have to do so that I can login? domain.com:8989 is blocked by my school’s firewall for being “unknown”, sub.domain.com isn’t.