This install script (second file) used to work months ago, but now it gives me an error (first file)

nix-install-multiuser.log

The user `nixbld1' is already a member of `nixbld'.
The user `nixbld2' is already a member of `nixbld'.
The user `nixbld3' is already a member of `nixbld'.
The user `nixbld4' is already a member of `nixbld'.
The user `nixbld5' is already a member of `nixbld'.
The user `nixbld6' is already a member of `nixbld'.
The user `nixbld7' is already a member of `nixbld'.
The user `nixbld8' is already a member of `nixbld'.
The user `nixbld9' is already a member of `nixbld'.
The user `nixbld10' is already a member of `nixbld'.
downloading Nix expressions from https://nixos.org/releases/nixpkgs/nixpkgs-16.09pre81560.c7f9b6c//nixexprs.tar.xz...
downloading https://nixos.org/releases/nixpkgs/nixpkgs-16.09pre81560.c7f9b6c//nixexprs.tar.xz...  [0/0 KiB, 0.0 KiB/s] [80/7153 KiB, 74.2 KiB/s] [256/7153 KiB, 106.0 KiB/s] [336/7153 KiB, 97.5 KiB/s] [384/7153 KiB, 85.2 KiB/s] [448/7153 KiB, 81.3 KiB/s] [512/7153 KiB, 76.4 KiB/s] [528/7153 KiB, 68.5 KiB/s] [576/7153 KiB, 65.3 KiB/s] [624/7153 KiB, 63.4 KiB/s] [640/7153 KiB, 58.5 KiB/s] [672/7153 KiB, 55.7 KiB/s] [688/7153 KiB, 51.8 KiB/s] [704/7153 KiB, 49.0 KiB/s] [736/7153 KiB, 47.8 KiB/s] [752/7153 KiB, 45.6 KiB/s] [768/7153 KiB, 43.7 KiB/s] [800/7153 KiB, 43.0 KiB/s] [848/7153 KiB, 43.1 KiB/s] [896/7153 KiB, 43.3 KiB/s] [944/7153 KiB, 43.4 KiB/s] [992/7153 KiB, 43.6 KiB/s] [1088/7153 KiB, 45.8 KiB/s] [1152/7153 KiB, 46.3 KiB/s] [1200/7153 KiB, 46.3 KiB/s] [1216/7153 KiB, 44.9 KiB/s] [1264/7153 KiB, 44.2 KiB/s] [1280/7153 KiB, 43.1 KiB/s] [1312/7153 KiB, 42.8 KiB/s] [1360/7153 KiB, 42.9 KiB/s] [1408/7153 KiB, 43.0 KiB/s] [1424/7153 KiB, 42.0 KiB/s] [1472/7153 KiB, 42.1 KiB/s] [1504/7153 KiB, 41.7 KiB/s] [1552/7153 KiB, 41.7 KiB/s] [1616/7153 KiB, 42.3 KiB/s] [1712/7153 KiB, 43.6 KiB/s] [1776/7153 KiB, 44.1 KiB/s] [1840/7153 KiB, 44.5 KiB/s] [1904/7153 KiB, 44.7 KiB/s] [1936/7153 KiB, 44.4 KiB/s] [1952/7153 KiB, 43.5 KiB/s] [2016/7153 KiB, 43.9 KiB/s] [2080/7153 KiB, 44.3 KiB/s] [2128/7153 KiB, 44.3 KiB/s] [2208/7153 KiB, 45.0 KiB/s] [2256/7153 KiB, 45.0 KiB/s] [2288/7153 KiB, 44.7 KiB/s] [2336/7153 KiB, 44.7 KiB/s] [2384/7153 KiB, 44.7 KiB/s] [2416/7153 KiB, 44.4 KiB/s] [2480/7153 KiB, 44.8 KiB/s] [2528/7153 KiB, 44.7 KiB/s] [2560/7153 KiB, 44.4 KiB/s] [2592/7153 KiB, 44.2 KiB/s] [2656/7153 KiB, 44.5 KiB/s] [2752/7153 KiB, 45.2 KiB/s] [2816/7153 KiB, 45.5 KiB/s] [2896/7153 KiB, 46.0 KiB/s] [2944/7153 KiB, 46.0 KiB/s] [2976/7153 KiB, 45.7 KiB/s] [3040/7153 KiB, 45.9 KiB/s] [3136/7153 KiB, 46.6 KiB/s] [3264/7153 KiB, 47.8 KiB/s] [3360/7153 KiB, 48.5 KiB/s] [3456/7153 KiB, 49.0 KiB/s] [3520/7153 KiB, 49.2 KiB/s] [3600/7153 KiB, 49.6 KiB/s] [3696/7153 KiB, 50.2 KiB/s] [3760/7153 KiB, 50.3 KiB/s] [3872/7153 KiB, 51.1 KiB/s] [3984/7153 KiB, 51.9 KiB/s] [4112/7153 KiB, 52.8 KiB/s] [4192/7153 KiB, 53.1 KiB/s] [4272/7153 KiB, 53.4 KiB/s] [4352/7153 KiB, 53.7 KiB/s] [4464/7153 KiB, 54.4 KiB/s] [4592/7153 KiB, 55.3 KiB/s] [4752/7153 KiB, 56.5 KiB/s] [4896/7153 KiB, 57.5 KiB/s] [4960/7153 KiB, 57.5 KiB/s] [5056/7153 KiB, 57.9 KiB/s] [5168/7153 KiB, 58.5 KiB/s] [5264/7153 KiB, 59.0 KiB/s] [5360/7153 KiB, 59.3 KiB/s] [5408/7153 KiB, 59.1 KiB/s] [5504/7153 KiB, 59.5 KiB/s] [5616/7153 KiB, 60.0 KiB/s] [5760/7153 KiB, 60.9 KiB/s] [5920/7153 KiB, 61.9 KiB/s] [6048/7153 KiB, 62.6 KiB/s] [6160/7153 KiB, 63.1 KiB/s] [6224/7153 KiB, 63.0 KiB/s] [6288/7153 KiB, 63.0 KiB/s] [6416/7153 KiB, 63.6 KiB/s] [6528/7153 KiB, 64.1 KiB/s] [6624/7153 KiB, 64.4 KiB/s] [6688/7153 KiB, 64.4 KiB/s] [6752/7153 KiB, 64.3 KiB/s] [6784/7153 KiB, 64.0 KiB/s] [6848/7153 KiB, 64.0 KiB/s] [6944/7153 KiB, 64.3 KiB/s] [7040/7153 KiB, 64.5 KiB/s]
unpacking channels...
upgrading nix-1.11.2 to nix-1.11.2
these paths will be fetched (60.95 MiB download, 230.00 MiB unpacked):
  /nix/store/0lx9vy36nnrqxr67njx1b4v5vyypjhli-boehm-gc-7.2f
  /nix/store/0md03indgzyngjxq34gbigiy109lk4gj-perl-5.22.1
  /nix/store/0mfa3mpl4740vvs49sxpk965bmzr2r40-openssl-1.0.2g
  /nix/store/0p2n7pzdx9df4qs8rcdr4wa6f4gg622c-sqlite-3.11.1
  /nix/store/1jis46bsynmhyb5bnbm91zw50sskamhf-nix-1.11.2-debug
  /nix/store/1sgzbykwmd7jbfgzrpd76zvc7qsiy4rf-openssl-1.0.2g-bin
  /nix/store/4jdz78898sk1rzhgdll9q7ha4hhi1s6l-perl-WWW-Curl-4.17
  /nix/store/4nax28wfmzgyma006rmczhk7lp6yaaad-perl-DBI-1.634
  /nix/store/5cc9pfza6s0kwql5ni23vsdk5hyl75jq-glibc-2.23-bin
  /nix/store/5nx94xmwy92cbkqk8fkhkn2za6xa5g9w-boehm-gc-7.2f-dev
  /nix/store/6cfi9xccwnbzr0p28ykflcq6345wnmjv-coreutils-8.25
  /nix/store/8q18iw2amk805ijrij1x7dbyym5xp36g-libsodium-1.0.10
  /nix/store/apsbzv3wc3wqv8mm3c0nk79a83gnawwz-curl-7.47.1-bin
  /nix/store/bdv0dy3kn36sp8wjj75ix9ikwi5qyi81-libssh2-1.7.0
  /nix/store/cl0ajhx1d5j5lg9q6zfh521cn1xq9lfm-bzip2-1.0.6
  /nix/store/cs3aymh7rx3b13nvyi9kvjkl5b101arr-gzip-1.6
  /nix/store/di3aja6hz4i01i6v2l0lm8gnxhfdwr52-xz-5.2.2
  /nix/store/f19mficl5msxfsbv4g7p61c4lyw7hl9d-gcc-5.3.0
  /nix/store/f69rq0mmyp5mycgr6h8x0l252vgmq5vd-gcc-5.3.0-lib
  /nix/store/fpnf809d1dzq4dy0m7clbgn97fm3xxyd-bash-4.3-p42
  /nix/store/fv8lbg8jk2amwsxzrw82r028r8fm3rl3-gnutar-1.28
  /nix/store/h76nc9049n3z81dpscrdki3km04f9ywj-perl-DBD-SQLite-1.48
  /nix/store/hjdnd35g6pk9500k5kvl44w8hcml2mi5-nix-1.11.2-man
  /nix/store/hsi8hvmnrynlgqz5scy2jbc7vdzf5imm-linux-headers-3.18.14
  /nix/store/iy0p96469x75k82igpym83c6aw67c1ky-libnghttp2-1.8.0
  /nix/store/jcm796kamqfcj3385cwymnkgxcz5951j-bzip2-1.0.6-bin
  /nix/store/lxpmkjnyqjiy06avd45svijgf953ckia-glibc-2.23-dev
  /nix/store/s34m1sphxxz3vzhlh04fw3g5fvwzb9aw-nix-1.11.2
  /nix/store/skdijd3r067cbhh5dn6ya83cz33and2w-xz-5.2.2-bin
  /nix/store/sw5w8nwya7nff3dhvy8gq02ay2blb6sj-nix-1.11.2-doc
  /nix/store/xcvj60a669vm2mzsh29d7kim1fhsidal-zlib-1.2.8
  /nix/store/xpip1s2ix1kw0xjgy33i14j4c8j37j0a-acl-2.2.52
  /nix/store/xx05sf7gg794ccr1cridjdd06nd0yiv8-curl-7.47.1
  /nix/store/yli5653cxg528sjmf2pc1i9nfvq79438-attr-2.4.47
  /nix/store/zsk34dk2wdzkryllmhfyh1bqkp77lsn6-glibc-2.23
fetching path /nix/store/sw5w8nwya7nff3dhvy8gq02ay2blb6sj-nix-1.11.2-doc...
fetching path /nix/store/hjdnd35g6pk9500k5kvl44w8hcml2mi5-nix-1.11.2-man...
fetching path /nix/store/zsk34dk2wdzkryllmhfyh1bqkp77lsn6-glibc-2.23...
fetching path /nix/store/hsi8hvmnrynlgqz5scy2jbc7vdzf5imm-linux-headers-3.18.14...

*** Downloading https://cache.nixos.org/nar/0hyii41ncz7znkphl6k8ya8i7m0cy0vbbddskhkgmbhxlaa7whr3.nar.xz to /nix/store/hsi8hvmnrynlgqz5scy2jbc7vdzf5imm-linux-headers-3.18.14...

*** Downloading https://cache.nixos.org/nar/1ddr2yr5h80qwqkcqilypvvj22mv2qvqbs54nxg0g4a37aknj2dc.nar.xz to /nix/store/hjdnd35g6pk9500k5kvl44w8hcml2mi5-nix-1.11.2-man...

*** Downloading https://cache.nixos.org/nar/0cldwbsbfbxdw96lchj9m3qlby46rjbrvx2r7h15zjy3s7869wdd.nar.xz to /nix/store/zsk34dk2wdzkryllmhfyh1bqkp77lsn6-glibc-2.23...

*** Downloading https://cache.nixos.org/nar/021nf1ikhyh75kqg28kyvbxgg0k597wqb1rfdzd6h8qqsrbf6xqz.nar.xz to /nix/store/sw5w8nwya7nff3dhvy8gq02ay2blb6sj-nix-1.11.2-doc...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (60) SSL certificate problem: unable to get local issuer certificate
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.
/nix/store/hs8lf8v9qqfs9m3jqmm97nqg7wg9dqjw-xz-5.2.1/bin/xz: (stdin): File format not recognized
error: unexpected end-of-file
download of https://cache.nixos.org/nar/1ddr2yr5h80qwqkcqilypvvj22mv2qvqbs54nxg0g4a37aknj2dc.nar.xz failed: No such file or directory
could not download /nix/store/hjdnd35g6pk9500k5kvl44w8hcml2mi5-nix-1.11.2-man from any binary cache
fetching path /nix/store/hjdnd35g6pk9500k5kvl44w8hcml2mi5-nix-1.11.2-man failed with exit code 1
killing process 12490
killing process 12493
killing process 12492
error: some substitutes for the outputs of derivation /nix/store/9z3cddsddjkldk7vra5lp3p2gxnda2ky-nix-1.11.2.drv failed (usually happens due to networking issues); try --fallback to build derivation from source 

nix-install-multiuser.sh

#!/bin/sh -e


main()
{
    if fresh_installation
    then
        prep_single_user_install
        install_single_user
    fi
    install_multi_user
    update_channel
    upgrade_default_profile
    install_chroot_profile
    upgrade_chroot_profile
    activate_chroot_building
}


fresh_installation()
{
    test ! -e /nix
}


prep_single_user_install()
{
    sudo mkdir /nix
    sudo chown "$(whoami):$(whoami)" /nix
}


install_single_user()
{
    curl https://nixos.org/nix/install | env --unset=NIX_REMOTE bash
}


install_multi_user()
{
    create_build_group
    create_build_users
    set_multi_user_perms
    install_nix_conf
    install_systemd_conf
    set_up_user_config
    export NIX_REMOTE=daemon
}


update_channel()
{
    /nix/var/nix/profiles/default/bin/nix-channel --update
}


upgrade_default_profile()
{
    sudo /nix/var/nix/profiles/default/bin/nix-env \
        --file ~/.nix-defexpr \
        --profile /nix/var/nix/profiles/default \
        --upgrade \
        --always \
        '*'
}


install_chroot_profile()
{
    sudo /nix/var/nix/profiles/default/bin/nix-env \
        --file ~/.nix-defexpr \
        --profile /nix/var/nix/profiles/chroot \
        --install bash
}


upgrade_chroot_profile()
{
    sudo /nix/var/nix/profiles/default/bin/nix-env \
        --file ~/.nix-defexpr \
        --profile /nix/var/nix/profiles/chroot \
        --upgrade \
        --always \
        '*'
}


activate_chroot_building()
{
sudo tee --append /etc/nix/nix.conf <<EOF > /dev/null
build-use-chroot = true
build-chroot-dirs = /dev /dev/pts /bin=/nix/var/nix/profiles/chroot/bin $(bash_nix_deps)
EOF
}


create_build_group()
{
    sudo addgroup --quiet --system nixbld
}


create_build_users()
{
    for i in $(seq 1 10)
    do create_build_user "$i"
    done
}


set_multi_user_perms()
{
    sudo chown --recursive root:root /nix
    sudo chown --recursive root:nixbld /nix/store
    sudo chown --recursive root:nixbld /nix/var/nix/daemon-socket
    sudo chmod 1775 /nix/store
    sudo rm --recursive --force /nix/var/nix/gcroots/per-user
    sudo mkdir --parents --mode=1777 /nix/var/nix/gcroots/per-user
    sudo rm --recursive --force /nix/var/nix/profiles/per-user
    sudo mkdir --parents --mode=1777 /nix/var/nix/profiles/per-user
}


install_nix_conf() {
    sudo mkdir --parents /etc/nix
sudo tee /etc/nix/nix.conf <<EOF > /dev/null
build-users-group = nixbld
build-max-jobs = 4
EOF
}


install_systemd_conf()
{
    sudo ln --symbolic --force \
        /nix/var/nix/profiles/default/lib/systemd/system/nix-daemon.service \
        /etc/systemd/system
    sudo ln --symbolic --force \
        /nix/var/nix/profiles/default/lib/systemd/system/nix-daemon.socket \
        /etc/systemd/system/sockets.target.wants
    sudo systemctl --system daemon-reload
    sudo service nix-daemon stop
    sudo service nix-daemon start
}


set_up_user_config()
{
    local profile_dir="/nix/var/nix/profiles/per-user/$(whoami)/default"
    ln --symbolic --force --no-target-directory "$profile_dir" ~/.nix-profile
}


create_build_user()
{
    local user_number="$1"
    local username="nixbld$1"
    sudo adduser --quiet --system --group --disabled-login \
        --home /dev/null  --no-create-home \
        --gecos "Nix build user $user_number" \
        "$username"
    sudo adduser "$username" nixbld
}


bash_nix_deps()
{
    /nix/var/nix/profiles/default/bin/nix-store --query --requisites \
            $(readlink -f /nix/var/nix/profiles/chroot/bin/bash) \
        | tr "\n" " "
}


main

I just ran into exactly the same problem, also with a nix-multi-user install script.