Sorted.
Please remove from script used to build Whonix:
WHONIX_APT_REPOSITORY_OPTS='--enable --repository stable'
export WHONIX_APT_REPOSITORY_OPTS
And as replacement, whenever a line says:
--build
Keep the --build but add also the following switch:
--redistribute
I.e. it should look like this:
--build --redistribute
After booting Whonix, please check that Whonix stable (buster!) repository is enabled and apt-get update is functional.
(This was required because of this.)
Whonix 15 build testing can start as per
However, functionality wise, it will take some more time.