This is not a Path MTU hole, Netalyzr is just telling you that between your end of the tunnel and the PoP there is a MTU of 1280. What is interesting is the next portion though "The path between our system and your network does not appear to handle fragmented IPv6 traffic properly." as that would indicate that some system on the path is dropping ICMPv6 Packet Too Big packets. As such, do check where that happens, thus check your firewall rules, ICMP is important. See the FAQ for the details. Note that sixxsd does not drop ICMP or any other kind of packet, especially as that would make tunneling fail quite horribly as every tunnel has a lower MTU than 1500. Do also note that one can change the MTU, see the FAQ for the details.