Nginx – Cannot connect to HTTPS port on Ubuntu

I've installed a new SSL certificate and set up Nginx to use it. But requests time out when trying to hit HTTPS on the site. When I telnet to my domain on port 80 it connects, but times out on port 443. I'm not sure if there's some defaults on Ubuntu preventing a connection. cURL also times out to the HTTPS address but responds to regular HTTP.

UFW status shows:

443  ALLOW    Anywhere

netstat -a shows:

tcp        0      0 *:https                 *:*                     LISTEN  

nmap localhost shows:

443/tcp  open  https

The relevant block in the Nginx config is:

server {
    listen 443;
    listen [::]:80 ipv6only=on;
    listen 80;
    root /path/to/app;
    server_name mydomain.com

    ssl on;
    ssl_certificate /etc/nginx/ssl/ssl-bundle.crt;
    ssl_certificate_key /etc/nginx/ssl/server.key;

    location / {
      proxy_pass http://mydomain.com;
      proxy_set_header Host $host;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

Edit: having tried a whole range of nginx configuration options, I'm really doubting it's the setup. If the ssl-bundle.crt is screwed up, would it cause the time out to happen? If so I can revert to PositiveSSL for support.