You two might want to compare your IPv6 settings. Try this through the Tools -> Run Cmd page:
Okay, here's the output that I see from the above command, hope that it helps. If I had source code, I could see what conditions cause the various log messages to come out or not, but I don't, and don't know if the relevant source code is even available. I've also replaced some of the address digits below with "x"'s, since I don't want addresses showing *smile*. Ipv6_rtr_addr below is the router's address.
I'm also assuming that when the OP upgraded to this new firmware that they cleared their nvram after upgrading and re-entered your settings manually. That's what I generally do, unless the versions are very close, and have few problems as a result:
ASUSWRT RT-N66U_3.0.0.4 Sat Jul 6 01:07:48 UTC 2013
admin@RT-N66U:/tmp/home/root# nvram show | grep ipv6
size: 34119 bytes (31417 left)
ipv6_6rd_ip4size=0
ipv6_tun_v4end=0.0.0.0
ipv6_accept_ra=1
ipv6_get_dns=2001:558:feed::1 2001:558:feed::2
ipv6_dhcppd=1
ipv6_ipaddr=
ipv6_state_t=0
ipv6_prefix=2601:9:xxxx:xx::
ipv6_ifdev=eth
ipv6_service=dhcp6
ipv6_tun_addrlen=64
ipv6_gateway=
ipv6_ifname=six0
ipv6_prefix_length=64
rc_support=mssid 2.4G 5G update usbX2 switchctrl manual_stb pwrctrl optimize_xbox ipv6 PARENTAL2 pptpd printer modem wimax openvpnd HTTPS webdav cloudsync appbase media diskutility nfsd
ipv6_ll_remote=
ipv6_dnsenable=1
ipv6_6rd_prefix=
ipv6_sbstate_t=0
ipv6_debug=0
ipv6_6rd_router=0.0.0.0
ipv6_dhcp_end=
ipv6_autoconf_type=0
ipv6_gw_addr=fe80::xxx:xxxx:xxxx:xxxx
ipv6_rtr_addr=2601:9:xxxx:xx:xxxx:xx:xxxx:xxxx
ipv6_dhcp_pd=1
ipv6_6rd_prefixlen=32
ipv6_tun_mtu=0
ipv6_tun_addr=
ipv6_tun_ttl=255
ipv6_dns1=
ipv6_dns2=
ipv6_dns3=
ipv6_get_domain=
ipv6_prefix_len_wan=64
ipv6_radvd=1
ipv6_dhcp_start=
ipv6_dhcp_lifetime=86400
ipv6_6rd_dhcp=1
ipv6_relay=192.88.99.1
admin@RT-N66U:/tmp/home/root#