521 error on domain and subdomain on different servers

Website, Application, Performance DNS & Network
1

Wonder if you would all be kind enough to offer assistance.

I have a domain (example.tld) and a subdomain (admin.example.tld) on different servers. Both have their own A record which are both proxied.

We have set up a login in panel through example frontend to access the admin.example

When I login from website domain to admin subdomain it gives a webserver is down 521 error when the SSL/TLS is set to Full or Full strict. When it is set to Flexible it works fine.

How can I get rid of the 521 error using full or full strict?

Many thanks in advance

2
  1. You should alway be using only Full Strict. Any of the other modes disable encryption and make your site insecure.
  2. The error means that your server cannot be reached by Cloudflare.
  3. Considering it works when you disable encryption, that suggests it should not be a network issue per-se.
  4. Either your HTTPS port is not properly configured or you have a firewall blocking access. Verify both.

Best you pause Cloudflare and make sure the site loads fine on HTTPS without Cloudflare. Once that works, it will also work with Cloudflare.

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