I cannot access the site over IPv4 or IPv6
I checked again. Now I am able to access the site over IPv4. However over IPv6 I am no longer able to get any response from the server.
Where I previously got connection refused over IPv6 I now get timeout. When looking at the network traffic I noticed something really weird. When I send TCP SYN packets to 2600:806:310::100 I get some packets back from a different IP address on that network. I actually receive neighbor solicitation messages from 2600:806:310::10.
Neighbor solicitation messages are only supposed to be sent to peers on directly attached links. I am nine hops away from that network, so there is no way I should receive those messages.
That's not a fault of the intermediate routers. They just forward the packet as they are supposed to without ever paying attention to the ICMPv6 payload. But the originating system shouldn't be sending me those neighbor solicitation messages.
The fact that I can still traceroute the IP address with no problems and the fact that the spurious messages originate from a different address in their network makes me think some load balancing system is involved. I am wondering if they could have made a mistake in the configuration of the netmask on some prefix causing the spurious neighbor solicitation messages.
But 2600:800::/27 and 2001:470::/32 only match on the first five bits. That would be a really strange misconfiguration. Did somebody perhaps configure a host to think it has a direct link to 2000::/3? Would a router actually forward the neighbor solicitation messages if they are send to an anycast MAC address?
The hop limit on the neigbor solicitation messages I receive make me think it must have been 255 when sent from the source.
I am tempted to reply to the neighbor solicitation messages with a MAC address of 33:33:00:00:00:02 just to see if that would actually make the communication with the webserver work.