Microsoft RPC over HTTPS

I'm struggling with the above with nginx as reverse proxy. I can see RPC_IN_DATA and RPC_OUT_DATA methods crossing the proxy, but get 503 served after a short time.

Config extracts:

gzip off;
tcp_nodelay on;
...
server {
listen 443;
location / {
proxy_buffering off;
proxy_pass http://backend-server:443;
}