nginx as reverse proxy with client ssl authentication

Hi,

I am using nginx 1.13.11 on Windows 10
I am trying to set up a reverse proxy for a HTTPS backend requiring client ssl authentication.

Tried this with the following config

---
worker_processes 1;


error_log logs/error.log debug;

events {
worker_connections 1024;
}


http {
include mime.types;
default_type application/octet-stream;



sendfile on;

keepalive_timeout 65;

server {
listen 443 ssl;
server_name localhost;

ssl_certificate C:/data/nginx-1.13.11/conf/crt;
ssl_certificate_key C:/data/nginx-1.13.11/conf/key;

ssl_session_cache shared:SSL:1m;
ssl_session_timeout 5m;

ssl_ciphers HIGH:!aNULL:!MD5;
ssl_prefer_server_ciphers on;

location /path {
proxy_pass https://backend
proxy_ssl_certificate C:/data/nginx-1.13.11/conf/backend.crt;
proxy_ssl_certificate_key C:/data/nginx-1.13.11/conf/backend.key;
}

}

}
---

This does not work. Debug log of a request can be found in the attached file.

When accessing the backend directly with importing the backend cert+key into Firefox works out of the box.

I do not know what software the backend is using.

Using an own apache with client authentication as a backend for my nginx reverse proxy, this given config works.

I have no clue how to solve this. Maybe its an SSL Problem between the nginx and the backend?

Does anyone has a hint how to get it work or how to analyse the reason of not working?