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
pgp4dDlvLVrpQ.pgp
Description: OpenPGP digital signature
