> Which version of the firmware do you use ? Is there something special on the Pi ?
there should not be anything special with the Pi, i have been using it for some time doing different things with it, i will try flashing it with a completely blank OS
> When using the [2a01:x] address in the address bar also ?
Yes, from all three (2a01:x, fdf5:x & fe80:x) addresses...
> From the Pi, can you ping all the ipv6 addresses of the win machine ? From the router ?
pi@raspberrypi:~$ ping -c 1 2a01:x:4129
PING 2a01:x:4129(2a01:x:4129) 56 data bytes
64 bytes from 2a01:x:4129: icmp_seq=1 ttl=64 time=1.36 ms
--- 2a01:x:4129 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.361/1.361/1.361/0.000 ms
pi@raspberrypi:~$ ping -c 1 fdf5:x:4129
PING fdf5:x:4129(fdf5:x:4129) 56 data bytes
64 bytes from fdf5:x:4129: icmp_seq=1 ttl=64 time=0.924 ms
--- fdf5:x:4129 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.924/0.924/0.924/0.000 ms
pi@raspberrypi:~$ ping -c 1 fe80:x:4129
connect: Invalid argument
Pinging the Windows machine from the Pi works on 2a01:x and fdf5:x address but not on fe80:x
root@Teltonika-RUT240:~# ping -c 1 2a01:x:4129
PING 2a01:x:4129 (2a01:x:4129): 56 data bytes
--- 2a01:x:4129 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss
root@Teltonika-RUT240:~# ping -c 1 fdf5:x:4129
PING fdf5:x:4129 (fdf5:x:4129): 56 data bytes
64 bytes from fdf5:x:4129: seq=0 ttl=64 time=3.111 ms
--- fdf5:x:4129 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 3.111/3.111/3.111 ms
root@Teltonika-RUT240:~# ping -c 1 fe80:x:4129
PING fe80:x:4129 (fe80:x:4129): 56 data bytes
64 bytes from fe80:x:4129: seq=0 ttl=64 time=1.835 ms
--- fe80:x:4129 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 1.835/1.835/1.835 ms
Pinging the Windows machine from the Router works on fe80:x and fdf5:x address but not on 2a01:x