having just spent several hours debugging the IPv6 setup on our OpenBSD VPS, I found out that it's not enough just to have the IPv6 addresses (as specified in the "Network information" page) assigned. It's also important that the link-local address is EUI-64 (i.e. based on the interface MAC address), or else various routing problems occur. OpenBSD 6.3 and later started using randomized addresses by default and I had weird problems since the upgrade.
I'd like to propose that this requirement (EUI-64 link-local address) be prominently displayed somewhere in the network information page, or at least the IPv6 setup documentation.