I was having issues connecting to the console as well. Fresh install. Messed with some SSL settings in ESET and it worked. Proxmox unchanged. Changed the settings back and it still works.
Random as hell. Can't say it builds faith in a product. Doesn't work, come back in a few hours and it does....
Like I said. That probably has nothing to do with Proxmox. Try disabling IPv6 and/or check you DNS.