This blog post claims:
https://grpugh.wordpress.com/2014/10/29/an-undocumented-change-to-captive-network-assistant-settings-in-os-x-10-10-yosemite/
the new URL is now:
http://captive.apple.com/hotspot-detect.html
instead of
http://www.apple.com/library/test/success.html
Do you support that change? Wont be able to test until Monday unfortunately.