Same here but it occured for me in Beta 2.
In beta 2 my captive network assistant brings up the portal page as a blank page with a dialog stating "A problem occurred - The webpage couldn't be loaded."
Opening Safari separately brings up the portal - just not the assistant. I've tried both http and https portal configurations and both connection types are broken.
Definitely still broken on beta 2. Tried Cisco and Aruba captive portal networks.
Broken for me too.
I've been able to get around it, possibly just luck, with two different captivate networks.
At a Hilton hotel, trying some other URL's in Safari caused the captivate page to come up in the full browser (maybe DNS highjack implementation?).
At a business' guest network that didn't work, but trying to have Safari go to 184.108.40.206 got me the captivate page. Again, might have just been luck but wanted to share.
Have the same error with the portal login/connect dialog boxes the OS opens up when you first connect and with the one Safari opens up when you try going to any site.
I've been using Chrome as a workaround to connect to them. No problems at all.
On one occasion, I got a connection, but Safari was claiming that the certificate of the Wifi provider was invalid.
Seems to be certificate related.
Perhaps being unconnected to the internet, Safari is unable to validate the certificate of the Wifi provider. :-)