I thought it had something to to with the substitute mechanism, but it also happens when I disable substituting.
~ $ guix pull --no-substitutes
Building from this channel:
building /gnu/store/54vrbwjqlc24z7ppw54y9ly0sczm5bkx-Python-2.7.15.tar.xz.drv...
Starting download of /gnu/store/jgav9ln0csn98rwj8f6v5nqs54yn2vv5-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
;;; 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
;;; 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
;;; 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
;;; Failed to autoload make-session in (gnutls):
;;; missing interface for module (gnutls)
error: make-session: unbound variable
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