If you’re getting an error along the lines of this in your syslog while trying to start a DHCPv6 server, you’ve probably got IPv4 ranges in your config file. It seems if it’s in IPv6 mode it can’t deal with IPv4 information being in the config file. :S
So, rip the IPv4 information out of your /etc/dhcp/dhcpd6.conf file and you should be good to go.