Here is an extract from the error 521 community tip.
seeing a 521 over HTTPS, it is possible that your origin server has not been configured to allow port 443 be accessed by Cloudflare IPs. In this case you should configure your server/firewall to being listening on port 443 and allow us to be able to conect.
I would suggest this is the issue here, and if you get a certificate on your server, I would expect it to work in Full (strict) mode.
Short answer:
Yes! 