I strongly suspect there to be some problem with the ftp client since
that's the second file that doesn't work using guix download but does work
using wget, on the same computer.

$ guix download ftp://ftp.denx.de/pub/u-boot/u-boot-2021.01.tar.bz2

Starting download of /tmp/guix-file.tORPhj
From ftp://ftp.denx.de/pub/u-boot/u-boot-2021.01.tar.bz2...
Throw to key `ftp-error' with args `(#<input-output: socket 16> "PASV" 425 "You cannot use PASV on IPv6 connections. Use EPSV instead.\r")'.
failed to download "/tmp/guix-file.tORPhj" from "ftp://ftp.denx.de/pub/u-boot/u-boot-2021.01.tar.bz2"
guix download: error: ftp://ftp.denx.de/pub/u-boot/u-boot-2021.01.tar.bz2: download failed