No, my Sophos is still not working with XS4ALL’s IPv6 implementation. I doubt that it will ever work.
So for the time being I have powered up my [OpenWRT](http://rutten.me/projecten/openwrt-en-xs4all-kpn) router again, which now handles the internet connection.
I have connected the WAN port of the Sophos SG125W to a LAN port on my OpenWRT router, and configured it as a DMZ-like setting. So all actual port forwards and firewall is still handled by the Sophos.
Not the best way, but at least I have my IPv6 working again.
Downside is that my outgoing IPv6 address is always the Sophos WAN address. Because my Sophos now acts like a gateway. Although all my clients are getting a valid address in my /64 range range, and are even reachable via the internet.