You will experience IPv6 issue with these two IPv6 subnets. This 2001:1900::/32 IPv6 subnet block has been assigned from ARIN to Level3, but Hurricane Circuit and Yahoo are inappropriately advertising out to the internet.
2001:1900:2254::/48 Currently advertised by Yahoo 2001:1900:2262::/48 Currently advertised by Hurricane Electric
To troubleshoot whether the websites is using this IPv6 address , you can use nslookup q=aaaa . Below is an example: