This error is caused by a bug[1] present on outdated versions of Linux (typically exhibited by DD-WRT and Tomato routers) and can also be fixed by upgrading to OpenWRT.

Or, if that is not an option, by creating an ip6tables rule:

  ip6tables -A PREROUTING -t mangle -p icmpv6 --icmpv6-type neighbor-solicitation -i `nvram get wan0_ifname` -d ff02::1:ff00:0/104 -j DROP

Or you can experiment with the ARP cache limits:

  sysctl net.ipv6.neigh.default.gc_thresh1=256
  sysctl net.ipv6.neigh.default.gc_thresh2=512
  sysctl net.ipv6.neigh.default.gc_thresh3=1024

[1] http://serverfault.com/a/461053