Unable to ping a specific system on IPv6

I've enabled IPv6 on my network, using DHCPv6 and subnet fd00::/64. My DHCP server's IP is fd00::1:1:1 and I'm unable to ping it from some systems on my network.

My radvd.conf:

interface br0
{
    AdvSendAdvert on;
    AdvManagedFlag on;
    AdvOtherConfigFlag on;

    MinRtrAdvInterval 3;
    MaxRtrAdvInterval 60;

    prefix fd00::/64
    {
        AdvAutonomous off;
        AdvOnLink on;
    };
};

IPv6 addresses on DHCP server:

# ip -6 addr list dev br0
4: br0:  mtu 1500 state UP qlen 1000
    inet6 fd00::1:1:1/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::223:5aff:fe5c:3035/64 scope link
       valid_lft forever preferred_lft forever

On client:

   IPv6 Address. . . . . . . . . . . : fd00::5(Preferred)
   Lease Obtained. . . . . . . . . . : 27 July 2016 19:37:38
   Lease Expires . . . . . . . . . . : 02 August 2016 22:13:23
   IPv6 Address. . . . . . . . . . . : fdf4:9ff3:68e4:3600:8c44:7b7d:8cd5:22b5(Preferred)
   Temporary IPv6 Address. . . . . . : fdf4:9ff3:68e4:3600:e10d:6a8d:d16f:ebf0(Preferred)
   Link-local IPv6 Address . . . . . : fe80::8c44:7b7d:8cd5:22b5%9(Preferred)
   IPv4 Address. . . . . . . . . . . : 10.0.0.5(Preferred)

Server is running Ubuntu 16.04.1, client is Windows 10.

I'm able to ping from server to client:

$ ping6 -w5 -c1 fd00::5
PING fd00::5(fd00::5) 56 data bytes
64 bytes from fd00::5: icmp_seq=1 ttl=64 time=0.245 ms

--- fd00::5 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.245/0.245/0.245/0.000 ms

But not client to server:

>ping -n 1 fd00::1:1:1

Pinging fd00::1:1:1 with 32 bytes of data:
Request timed out.

Ping statistics for fd00::1:1:1:
    Packets: Sent = 1, Received = 0, Lost = 1 (100% loss),

I can ping the link-local address though.

Client can ping other IPv6 systems on my network:

>ping -n 1 fd00::1

Pinging fd00::1 with 32 bytes of data:
Reply from fd00::1: time


It's not being blocked by iptables on server (raw, mangle and nat tables are also empty with policy set to ACCEPT):

# ip6tables -n -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

I've spent a few days trying to figure this out, but not got anywhere. Has anyone got any ideas?

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