Def convinced its a Safari issu...
Steps...
Join guest network, open up safari and login/create account
Then get the safari error about insecure connection to server
If I then try Chrome.. it works fine, no errors.. just log in and away I go... If I then try Safari, this then works - obvioulsy as Ive then passed the auth stages and been granted access.
Viewing the portal login page in chrome, shows a valid (green) cert which has been verified by DigiCert SHA2 Secure Server CA. I can then login and get access to the internet.
Try to view the same in Safari, and get the error saying it cant establish a secure conneciton to the server.... so the switch to Chrome and log in!