MacOS Mojave 10.14.1 Captive Wi-Fi Problem This is the same/similar to this one, but I'm running the final version of 10.14.1. I can make the problem go away if I remove the following files from /Library/Preferences/SystemConfiguration/:


com.apple.airport.preferences.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist


After a reboot, the captive login window works, but at some point, it stops working again and looks like: 

I haven't tested much - I don't have regular access to many captive Wi-Fi networks.

I can work around the problem by dismissing the error, closing the window and using Safari to go to https://captive.apple.com, but I would like to solve the problem rather than work around it if possible.

Has anyone figured this out or can anyone point me in the right direction?
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-