IPv6 routers do not fragment packets - unlike IPv4. So if one side chooses too big of IPv6 packet then it won't make it to the other side. IPv6 is supposed to choose the lowest MTU along a path but it's better to set the lower MTU yourself.
I'm not saying this is an issue here but a general comment..
With IPv6 it is critical for path MTU discovery to work properly. Problem is too many break path MTU discovery within the IPv4 world with wrong firewall rules never mind with IPv6 where too many people use the same set of wrong firewall rules and the pain is much much greater. Too many people foolishly block all ICMP and that is wrong. But they seemingly like having a broken network I guess.
