LOGIN at TreasuryDirect.gov fails with ipv6

Logging into TreasuryDirect.gov on any of my computers no longer works unless I disable ipv6 in my Ethernet adapter. For my phone, I can't get there if connected to my home network, but no problems if using my cellular carrier.

Something has happened at comcast with ipv6. If I disable ipv6 inside the Control Panel --> Network and Internet --> Network connections, my ability to LOG into TreasuryDirect.gov is instant. Otherwise, it is inaccessible in all browsers, except - oddly - Mozilla Firefox. 

This is critical as TreasuryDirect is where everyone buys and sells their government Treasury bonds.

To be clear, you can GET to the website home page, but when you hit the LOGIN button, it fails under ipv6.

https://www.treasurydirect.gov/go_to_login.htm