I went through the same problem today and found that (at least in my case) it was the lack of TLS v1.3
I had just made a server using nginx + php-fpm and a self signed ssl to use below CloudFlare proxy.
When I switched from the production server to this new one, it gave error 525.
I gave the command: curl -I https://your_server_public_ip/
and it returned the error:
error: 1408F10B: SSL routines: ssl3_get_record: wrong version number
This error is described in the CloudFlare community at:
https://community.cloudflare.com/t/community-tip-fixing-error-525-ssl-handshake-failed/44256
There they advise turning off TLS v1.3 on the CloudFlare panel, but I decided to try installing it.
Using nginx is so easy that I don’t know why to have it shut down.
Only add TLSv1.3 like this-> ssl_protocols TLSv1.2 TLSv1.3;
in your nginx/snippets/ssl-params.conf
file (default Ubuntu 20 and 18) that will work and you still use the latest and most secure protocols.