I have no idea where the qxdisplay.desktop error comes from, I can't see a qxdisplay.desktop file being called anywhere in the lxc-templates or harbour-containers source files. I suspect it's some other UNIX tool getting in the way somewhere and automatically expecting a desktop file?

Was the container started with the LXC-attach button when you ran the following command?

Can you try that one instead where I change the display?

In case you haven't, you should start the container first, attach to it using LXC-attach, and then in a different terminal window, as defaultuser, use that above command.

If your containers are not configured yet, I would indeed start fresh after destroying them and creating a new Debian Sid container. Sometimes things can break with Debian Sid because packages change fast and this version is not made to be stable (at some point the Xwayland compilation was broken for instance). However we tested a few days ago with Mosen on Pro1 and he could install Debian Sid (from CLI only using the instruction in the lxc-templates wiki for now, still issues with creating containers from GUI) and then setup the X desktop using the harbour-containers button at the bottm, and then start X with the above command.

I updated the daemon.py fork, be sure to use the latest version before next attempts.