Bug#642454: Re: Bug#642454: dpkg-deb: file [file] contains ununderstood data member data.tar.xz, giving up
this bug can be closed. I had dpkg pinned at an old version. After updating dpkg, it now works. I'm sorry for the inconvenience caused.
PS: I used an anonymous mailbin for the original bugreport. I'm the original bug reporter, this time from a proper e-mail address.