Yeah as I initially posted, I don't think the things you're missing are mandatory. It's worth digging further at least!

Another thing to test: @mibry on Discord observed that there has been commits to harbour-containers after the latest .rpm release, including one that has to do with the container installation. From what I see in the commit history, this should only influence XFCE setup and not the issues you're facing, but it doesn't hurt to fix that before we forget.

Can you guys try to install the latest harbour-containers release (as we did before), then replace daemon.py with the one on my fork (same as before too), and then also replace the content of /usr/share/harbour-containers/scripts/guest/setups/debian.sh by that of this version, and only after that try to create a new Debian Sid container? Make sure both daemon.py and debian.sh are executable (devel-su chmod +x /path/to/file). If my fork breaks things and creates new bugs you don't observe with vanilla daemon.py, please report and tell if your user is named nemo or defaultuser, you device and SFOS version, and whether you have an active symlink between /home/nemo and /home/defaultuser.

If your harbour-containers is already showing an empty GUI and you can't interact with it, then destroy your existing Linux containers using lxc-destroy as described in a post above (be sure to backup your rootfs from /home/.lxc and config file from /var/lib/lxc before if your container has worked in the past and has some value to you).