Www for subdomain

As i wrote in my first post:

Free SSL on this domain will not work

NET::ERR_CERT_COMMON_NAME_INVALID

The SSL Certificates Cloudflare proivides for free only work on the 2nd (.icyberzone.com) and 3rd level forum.icyberzone.com). The 4th level is not covered.

Since https://forum.icyberzone.com/ works well, you could try to redirect permanently (HTTP 301) from www.forum. to forum. But i am not suere if it solves your SSL error.
Try it with page rules.

Something like