[SOLVED] Struggling to get IPv6 working on LXC

[Pixels]

Hello,
I've got one box with a single IPv4 and a /64 IPv6 block, but I am not able to configure it correctly.
I've searched in dozens of webs for a long time but no success.

What I want is to assign a CT its dedicated IPv6 address. IPv4 is now working as NAT so there is no problem in that aspect.

Here is my /etc/network/interfaces:

auto lo
iface lo inet loopback

iface lo inet6 loopback

auto eth0
iface eth0 inet static
        address  (public_ip)
        netmask  255.255.255.0
        gateway  (gateway)
        post-up echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
        dns-nameservers 8.8.8.8 8.8.4.4

iface eth0 inet6 static
        address  XXXX:XXXX:XXXX:XXXX::2
        netmask  128
        gateway  fe80::1
        pre-up modprobe ipv6
        autoconf 0
        post-up echo 1 > /proc/sys/net/ipv6/conf/all/proxy_ndp
        dns-nameservers 2001:4860:4860::8888
        up sysctl -p

auto vmbr0
iface vmbr0 inet static
        address  192.168.2.1
        netmask  255.255.255.0
        bridge_ports none
        bridge_stp off
        bridge_fd 0
        post-up echo 1 > /proc/sys/net/ipv4/ip_forward
        post-up iptables -t nat -A POSTROUTING -s '192.168.2.0/24' -o eth0 -j MASQUERADE
        post-down iptables -t nat -D POSTROUTING -s '192.168.2/24' -o eth0 -j MASQUERADE

iface vmbr0 inet6 static
        address  XXXX:XXXX:XXXX:XXXX::1
        netmask  64

Box IPv6 is working, but any CT that I create gets no IPv6 connection at all. I've tried several IPv6 vmbr0 configs but none of them worked.
Any clue on this?

PD: No OVH, so is not a problem of virtual macs.