To try to trace this I would suggest the following:
- boot with 'install url=http://<full-url-to-preseed-file>'
- at the first dialog, switch to VT2
- edit /var/lib/dpkg/info/network-preseed.postinst and add a 'set -x'
- continue the installation untill the problem occurs
- find some way to get the syslog off the system
The last may be tricky. You could try booting with BOOT_DEBUG=3 and
editing /etc/inittab so the debian-installer program does not
automatically respawn.
If you add modules=openssh-client-udeb at the boot prompt, you will be
able to run scp from a shell.
Cheers,
FJP