For some reason, suddenly, IPv6 on my Windows 2008 R2 system stopped working.

I wasn't able to ping or trace route with the following commands:

    ping -6 www.google.com
    tracert -6 www.google.com

When I used route -6 delete * to delete the routing table, followed by disabling and re-enabling IPv6 protocol on the adapter, it worked for a while, and then stopped.

Eventually, I figured out there were stale entries in the IP configuration, and a simple ipconfig /release6 * resolved it. I was then able to ping and trace route on IPv6 to www.google.com.

It seems that Windows had a bug with regards to IPv6.