Seeing the exact same error on Nginx 1.9.0 and it looks like it was caused by the HTTPS endpoint using SNI.
Adding this to the proxy location fixed it:
proxy_ssl_server_name on;
- https://en.wikipedia.org/wiki/Server_Name_Indication
- http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_server_name