@Joseph-TP I was contacted by a support engineer. He couldn't fix the problem and took some network dumps. I'm not sure, what's happening now. But I hope he comes up with a solution.

Hello @Joseph-TP, thanks for your answer. I have confirmed the IPv6 informations with my ISP. I've also talked to their support. We've already tried the "non-address" option. The topology is, as you...

I have problems to assign IPv6 addresses in my network. My provider is Atomic South Africa (in combination with Openserve) and yes, they provide IPv6 (Google: atomic ipv6 howto). I connect via PPPoE...