Do both servers support ipv6 and so you have allowed access for ipv4 or ipv6 but not the other? Though you mentioned using the IP, so that should rule that out but to be safe I would check on that.
Are Blesta server and Cyberpanel server separate? If not, adding 127.0.0.1 might be needed also.
And finally, can you curl to the cyberpanel url and get some sort of response?
curl -v https://server.example.com:8443/
That should output the page with headers and such. If you can connect to it that way, then at least the firewall isn't blocking and is either login info or the allow list.