Additional information to reproduce the problem:
1) stretch install is started with pxelinux with this parameters:
LABEL v
MENU LABEL (^v) vga console amd64
kernel linux/debian/stretch/9.0/amd64/linux
append initrd=linux/debian/stretch/9.0/amd64/initrd.gz
patchlevel=20170618T160104Z BOOT_DEBUG=2 DEBCONF_DEBUG=5
log_host=192.168.120.22 log_port=554 ipv6.disable=1 language=en country=DE
locale=en_GB.UTF
-8 keymap=de url=http://preseed.company.tld/linux/debian/preseed/stretch/x.cfg
--- ipv6.disable=1
2) the preseed file x.cfg looks like this
# time/zone
d-i time/zone select Europe/Berlin
tzsetup-udeb time/zone select Europe/Berlin
tzsetup-udeb tzsetup/country/DE select Europe/Berlin
# Participate in the package usage survey?
popularity-contest popularity-contest/participate boolean false
# skip creation of a normal user account.
d-i passwd/make-user boolean false
# lang & keyboard
d-i debian-installer/language string en
d-i debian-installer/country string DE
d-i debian-installer/locale string en_GB.UTF-8
d-i localechooser/supported-locales multiselect en_US.UTF-8, de_DE.UTF-8
d-i keyboard-configuration/xkb-keymap select de
### Mirror settings
d-i mirror/protocol string http
d-i mirror/country string manual
d-i mirror/http/hostname string snapshot.debian.org
d-i mirror/http/directory string /archive/debian/20170618T160104Z
d-i mirror/http/proxy string http://proxy.company.tld:8000
3) The syslog output of the installer with DEBCONF_DEBUG=5 kernel command line
Here are the last lines just before failing
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; --> METAGET
base-installer/debootstrap/error/couldntdl description
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; <-- 0
Debootstrap Error
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; --> SUBST
base-installer/debootstrap/error/couldntdl SUBST0
dists/stretch/main/binary-amd64/Packages
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; Adding [SUBST0]
-> [dists/stretch/main/binary-amd64/Packages]
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; <-- 0
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; --> INPUT
critical base-installer/debootstrap/error/couldntdl
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; <-- 0 question
will be asked
Jul 6 14:21:44;debian-stretch.company.tld;user.debug;debconf:; --> GO
The complete syslog output can be found here
https://cloud.aschendorff.de/s/QrEBjzW8txqHFwN
