@Sasha nixos.org has three nameservers, all in the same AS (bad practice). This AS is not reachable from Free in IPv4 (it works fine over IPv6). https://gist.github.com/bortzmeyer/c0549be23c9971bb2d71c5146dff9060
It means the resolver you use does not have IPv6 (bad).
And it means that the Utag people should investigate and talk with Free.