MDS nailed it in the comment section, essentially when your are getting a 521 error on SSL requests, but non-SSL requests work fine; it usually indicates one of two things:
- Your web server is not set-up to receive SSL connections.
- Your firewall is blocking connections over SSL.
This behaviour can occur in Full and Full (Strict) mode, but not Flexible as the connection will be in plain-text HTTP.
If your web server is not configured to receive SSL connections, you will need to add this configuration before using Full or Full (Strict), however if it is configured you will instead need to whitelist port 443 on your server.
In IP Tables it is possible to open up port 443 as follows:
iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT
The next step would be to whitelist CloudFlare's IPs, there is a guide on how to whitelist CloudFlare IPs in IPTables.