I have very similar issue: this part of preseed fails (did not fail in stretch), with error message - Could not reach repository (options are retry, modify or ignore - or something similar) (in syslog it says - Repository not signed) d-i mirror/country string manual d-i mirror/http/hostname string 10.88.0.1 d-i mirror/http/directory string /debian-10-amd64 d-i mirror/http/proxy string
what helps little bit is to ignore the above error manually, which will just comment above repository in sources.list (# deb http://10.88.0.1/debian-10-amd64 buster main) and following line in preseed: d-i apt-setup/local0/repository string [trusted=yes] http://10.88.0.1:80/debian-10-amd64 stable main the installation finishes fine then
