+# --debootstrap-options --no-merged-usr \Are you sure this diff, starting with the comment character, really enables that option? Or is this a later diff after you've concluded it was not effective and re-disabled it?
Later on I deleted that line, because it turned out that this option is not available in the package of debootstrap available in stable. The error remains.
There's one difference that I forgot to mention: I used to build an i686 image from an i686 virtual machine, while now I'm trying to build it from an amd64 container. I tried to change the architecture of the image to amd64 to see if that was the issue. It seems that it isn't.
Perhaps there's some conflict between systemd-nspawn - which is running the debian container where I execute live-build (as my host is running Fedora) - and live-build or better other debian components?