Deleting ~/.local/share/harbour-containers/, /home/.lxc/Debian and /var/lib/lxc/… and then reinstalling the application has solved the problem and the Containers app now works normally.
I had config files left /var/lib/lxc and I needed to delete those as well to get the app working again.
Thank you very much to you guys for all the help!
Edit: Grammar
I have a slightly different problem, which I also reported on github. I had several containers running without any problems, until I got “Could not load page” when tapping on a container. I removed the app and also the folders mentioned in the solution to restart with a clean slate, but to no avail…
Check what running from cli gives you, there should be a hint there what’s going wrong
Ok, the cli output led me to the issue. I had played around with loads of qlm files to speed up animations and there was a problem in my modified /usr/lib64/qt5/qml/Sailfish/Silica/ExpandingSectionGroup.qml. After restoring the backup of this file I had made, the problem went away. Thanks!