If you are using nginx behind your loadbalancer then it's important that the default_server is returning 200 or some other 2**. That means that if you for example have a rewrite rule that returns 301 then it will fail.
The solution is to set default_server on your main server:
server {
# Rewrite calls to www
listen 443;
server_name example.com;
return 301 https://www.example.com$request_uri;
}
server {
listen 443 default_server;
server_name www.example.com;
...
