MacOS Mojave 10.14.1 Captive Wi-Fi Problem
It may be related to the Sophos antivirus scanner, see https://support.home.sophos.com/hc/en-us/articles/360020126572-Captive-Portal-Login-Page-does-not-load-on-MacOS-when-connecting-to-a-Public-Guest-Wi-Fi-Hotspot-