I thought it had something to to with the substitute mechanism, but it also happens when I disable substituting.
~ $ guix pull --no-substitutes
Updating channel 'guix' from Git repository at 'https://git.savannah.gnu.org/git/guix.git'...
Building from this channel:
  guix      https://git.savannah.gnu.org/git/guix.git 5e741ff
building /gnu/store/54vrbwjqlc24z7ppw54y9ly0sczm5bkx-Python-2.7.15.tar.xz.drv...

Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz
From https://www.python.org/ftp/python/2.7.15/Python-2.7.15.tar.xz...
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable

Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz
From https://mirror.hydra.gnu.org/file/Python-2.7.15.tar.xz/sha256/0x2mvz9dp11wj7p5ccvmk9s0hzjk2fa1m462p395l4r6bfnb3n92...
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable

Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz
From https://berlin.guixsd.org/file/Python-2.7.15.tar.xz/sha256/0x2mvz9dp11wj7p5ccvmk9s0hzjk2fa1m462p395l4r6bfnb3n92...
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable

Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz
From https://tarballs.nixos.org/sha256/0x2mvz9dp11wj7p5ccvmk9s0hzjk2fa1m462p395l4r6bfnb3n92...
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable

Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz
From https://archive.softwareheritage.org/api/1/content/sha256:22d9b1ac5b26135ad2b8c2901a9413537e08749a753356ee913c84dbd2df5574/raw/...
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable
failed to download "/gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz" from "https://www.python.org/ftp/python/2.7.15/Python-2.7.15.tar.xz"
builder for `/gnu/store/54vrbwjqlc24z7ppw54y9ly0sczm5bkx-Python-2.7.15.tar.xz.drv' failed to produce output path `/gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-Python-2.7.15.tar.xz'
build of /gnu/store/54vrbwjqlc24z7ppw54y9ly0sczm5bkx-Python-2.7.15.tar.xz.drv failed
View build log at '/var/log/guix/drvs/54/vrbwjqlc24z7ppw54y9ly0sczm5bkx-Python-2.7.15.tar.xz.drv.bz2'.
cannot build derivation `/gnu/store/jwhmsg7wnlszafb8nqzccvfpa588i046-Python-2.7.15.tar.xz.drv': 1 dependencies couldn't be built
building /gnu/store/qrjxkbas3jb4gwsv39ni2zzilbwwpck4-binutils-2.23.2.tar.xz.drv...
cannot build derivation `/gnu/store/cz4sr5whzf3qhp8c47bzlfzcxxwl485i-python2-2.7.15.drv': 1 dependencies couldn't be built
guix pull: error: build failed: build of `/gnu/store/cz4sr5whzf3qhp8c47bzlfzcxxwl485i-python2-2.7.15.drv' failed