Full (strict) SSl cloudflare

Question

I am trying to install Full (strict) SSL on cloudflare.

For this I:

1. created a certificate in cloudflare and connected it to nginx
2. enabled Authenticated Origin Pulls in cloudflare
3. enabled Full (strict) SSL

The nginx "Welcome to nginx" base page began to open on the site. As soon as I change ssl to flexible, everything immediately works correctly

nginx error logs are empty What could be the problem?

my default nginx conf:

server {
  listen 443 ssl;
  listen [::]:443 ssl http2;

  server_name site.com;

  ssl_certificate /etc/ssl/cert.pem;
  ssl_certificate_key /etc/ssl/privkey.pem;
  ssl_client_certificate /etc/ssl/cloudflare.crt;
  ssl_verify_client on;
}