Hetzner has a strange IPv6 routing setup where the default gateway is not in the same subnet as the host. Clearly, this is to avoid wasting space on glue nets, but it does not look pretty. Obviously, Linux just ignores the fact that it isn’t supposed to work, but no such luck on FreeBSD. As Bitmand wrote some time ago, it can be solved with a static route but the FreeBSD startup scripts, apply the default route before static routes, leaving it no other option but to reject the default route as it has no route to its target at that time. Instead of hacking the startup scripts, this can be solved easier with two static routes with fixed order, which is still a hack though not as ugly and less error prone when upgrading the system files. For reference, especially for myself next time I need this, here’s my full ipv6 startup configuration:

rc.conf:
ipv6_enable="YES"
ipv6_default_interface="re0"
ipv6_static_routes="defgw def"
ipv6_route_defgw="2a01:XXXX:XXXX:XXXX::1/59 -prefixlen 59 -iface re0"
ipv6_route_def="default 2a01:XXXX:XXXX:XXXX::1"
ipv6_ifconfig_re0="2a01:YYYY:YYYY:YYYY::2/64"


systctl.conf
net.inet6.ip6.accept_rtadv=1