Xfinity is slowly changing over their IPv6 configurations from having the DHCP6 server answer from a link local address to being forwarded from a remote address. You may have run into this when you rebooted the router.
If your IPv6 firewall has a statement like this from the default config
add action=accept chain=input comment="defconf: accept DHCPv6-Client prefix delegation." dst-port=546 protocol=udp src-address=fe80::/10
Try changing it to allow remote forwarded DHCPv6 like this
add action=accept chain=input comment="accept DHCPv6-Client prefix delegation only from LL relay" dst-address=fe80::/10 dst-port=546 protocol=udp
Earlier Xfinity discussion:
http://forum.mikrotik.com/t/xfinity-comcast-dhcpv6-configuration-change/156031/1
Edit: fixed url