I just tried flyctl ssh console for a London machine, via a European VPN, and it was fine.
The address in your error appears to be part of the IPv6 infra at Fly. This might be related: Wireguard nodes (*.gateway.6pn.dev) unaccessible from IPv6 only
Try a different connection e.g. pairing with your phone. I wonder if there is something up with your current connection.