Testing from a Servepath hosted server via above.net, it looks like a HE 6to4 relay router failed on Friday around 2011-10-21T17:47 UTC.
This is a Windows server and Windows own traceroute reports it like this (non-6to4 IPv6 destinations fail too):
C:\Users\Me>tracert 192.88.99.1
Tracing route to 192.88.99.1 over a maximum of 30 hops
1 19 ms 5 ms 2 ms 173.1.128.17.reverse.gogrid.com [173.1.128.17]
2 <1 ms <1 ms <1 ms vl22.border-core1.sfo2.servepath.com [208.96.31.
1]
3 1 ms 1 ms 1 ms xe-5-3-0.er2.sjc2.us.above.net [64.124.196.5]
4 1 ms 1 ms 1 ms 64.125.28.13.available.above.net [64.125.28.13]
5 1 ms 1 ms 1 ms xe-1-1-0.mpr3.sjc7.us.above.net [64.125.27.89]
6 8 ms 14 ms 3 ms 10gigabitethernet2-3.core1.sjc2.he.net [206.223.
116.37]
7 * * * Request timed out.
8 4 ms 4 ms 4 ms 192.88.99.1
Trace complete.
C:\Users\Me>tracert -6 2002:c058:6301::1
Tracing route to 2002:c058:6301::1 over a maximum of 30 hops
1 Destination protocol unreachable.
Trace complete.
