Nginx reverse proxy HTTPS to HTTP IIS backend

I have ASP.NET app hosted at backend IIS (Windows Server 2012 R2) and Debian with Nginx open for public with SSL certificates and so on.

Nginx proxying http://myexample.com request to HTTP backend without any problems with config:

server {
       listen 80;
       server_name my.example.com

       location / {
                  proxy_pass http://backend:1234;
       }
}

But when I try https://my.example.com – web browser gets link https://my.example.com/Login.aspx (which is correct) and then 404 Not Found.

HTTPS Nginx config (I tried with commented lines as well but still 404):

server {
       listen 443 ssl http2;
       server_name my.example.com

       -- SSL part ---

       location = / {
                    proxy_pass http://backend:1234;

                    #proxy_set_header Host $host;
                    #proxy_set_header X-Real-IP $remote_addr;
                    #proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                    #proxy_set_header X-Forwarded-Proto $scheme;
                    #proxy_redirect http:// $scheme://;
        }
 }

Nginx log show this:

[error] 27936#27936: 1* open() "/etc/nginx/html/Login.aspx" (2: No such file or directory), client: IP, server: my.example.com, request: "GET /Login.aspx HTTP/2.0", host: "my.example.com"

Why isn't Nginx proxying GET requests to backend server?