I'm using Falkon 3.0.0 on a Gentoo Linux system. I noticed Falkon still generates the ~/.cache/qupzilla directory even after the name change. By the way the ~/.cache/falkon directory exists on my system but it's empty.
Run: sed -i.bak -e 's/qupzilla/falkon/g' ~/.config/falkon/profiles/default/settings.ini
I had CachePath=/home/fturco/.cache/qupzilla/default in /home/fturco/.config/falkon/profiles/default/settings.ini. Your command fixed the problem. Thank you.