Hi there @volkanalkilic,

It sounds like that there might be an issue with your Apache or Nginx web server (depending on which one you are using), I would recommend going through these answers here in order to get some more information regarding the actual problem:

• If you are using Apache:

https://www.digitalocean.com/community/questions/how-to-troubleshoot-common-apache-issues-on-linux-server

• If you are using Nginx:

https://www.digitalocean.com/community/questions/how-to-troubleshoot-common-nginx-issues-on-linux-server

Another thing that could be causing the issue is if the firewall on your Droplet is blocking Cloudflare if this is the case you need to make sure that you whitelist CloudFlare’s IP ranges so that this does not happen in the future.

You can find Cloudflare’s IP rages here:

https://www.cloudflare.com/ips/

Hope that this helps!
Regards,
Bobby

Error 521 occurs because the origin web server refuses a connection from Cloudflare. More specifically, Cloudflare tried to connect to your origin server on port 80 or 443 but received a connection refused error