People keep asking me why installing kernel-images from backports.org fails, and what's the workaround. The reason why installing the packages fails is a broken yaird backport. Because nobody seems interested in fixing it, I just removed yaird from the backports.org archive. Use initramfs-tools instead, it works fine.
Thanks to my new employer (for those who didn't notice, I left Teamix and work for IP Exchange since September), this server has a native IPv6 connection since I moved it to it's new colocation about two weeks ago.