It looks like you're using Cloudflare's Origin CA service, nice! The issue looks like you've put your SSL private key in the ssl_client_certificate attribute and not put your real SSL certificate in your configuration. Your Nginx SSL configuration should contain the following lines instead:
27.08.2018 · If your server is running with Nginx 1.15.0 or a newer release, you can remove the line ssl on; Reload your nginx configuration with nginx -t && service …
01.10.2020 · Configure your server and Nginx with the Cloudflare SSL After the certificate files are created, now we need to configure origin cloudflare ssl with nginx, meaning – we need to tell nginx to use the cloudflare origin ssl that we generated earlier . Again with the text editor open the Nginx server configuration for your website.