I worked out the reason why these errors were appearing…
2019/06/02 06:31:53 [error] 8204#8204: *14 connect() failed (111: Connection refused) while connecting to upstream, client: ::ffff:192.168.10.1, server: mydynhost.ddns.net, request: "
GET /api/config HTTP/1.1", upstream: "http://[::1]:8123/api/config", host: "mydynhost.ddns.net:11111"
in my sites-enabled/default config file I was referencing the Hass instance by LOCALHOST. Changing to 127.0.0.1 solved the issue. Localhost resolves to an IPv6 address when responding to an incoming IPv6 from my iphone etc. By setting to 127.0.0.1 ensures we go to the IPv4 instance of HA. (as HA doesn’t do dual stack at present)
location / {
proxy_pass http://127.0.0.1:8123;
proxy_set_header Host $host;
proxy_redirect http:// https://;
proxy_http_version 1.1;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $connection_upgrade;
}