tunnel up but no traffic coming back

[adieball]

Hi

My tunnel seems to be up, but no traffic is coming back.

Code Select Expand

tun0             2001:470:6c:363::2/64             u/u  HE.NET IPv6 Tunnel

My config (vyos / Vyatta)

Code Select Expand

tunnel tun0 {
        address 2001:470:6c:363::2/64
        description "HE.NET IPv6 Tunnel"
        encapsulation sit
       
        local-ip 10.2.2.1
        multicast disable
        remote-ip 216.66.86.114


I cannot ping the IPv6 address of my tunnel endpoint, but I can ping the IPv4 address, so it seems to be up.

Using shark I can see traffic going out but nothing coming back.

The Router is connected to a FritzBox (7490). I disabled IPv6 Support on it as it looks like this is only for IPv6 done by the FritzBox itself (6in4, 6RD, Tunnel). Connected to Deutsche Telekom DSL.
In fact, it was working fine for a long time and I'm not aware that I changed anything.
The (public) IP of my Tunnel Endpoint is correct (on the HE Tunnel site), checked this already.

Any additional things I can do to troubleshoot this issue?

Thanks