If you get this error message in your console (dmesg) logs on Debian or Ubuntu:
IPv6 addrconf: prefix with wrong length 96
Then what is happening is that your server is autoconfiguring an ipv6 address that is also in use somewhere else.
You can fix this by doing the following:
You will also need to manually add those entries to /etc/sysctl.conf also to have them survive a reboot.