Finally I managed to make it work. Linux was a no go. Tried with my main distro, Pop!_OS. Didn,t work. Tried with Ubuntu 20.04, didn´t work. Bought a new USB-cable and tried it on my MacBook from 2008 again, didn´t work. Then I tried it on my Lenovo T440p Hackintosh with MacOS Mojave. It was´t possible to install Homebrew using the Jolla instructions. This however worked:
Installing fastboot tools didn´t worked as per the instructions, “brew tap caskroom/cask” had to be swapped out for “brew tap homebrew/cask”
“brew cask install android-platform-tools” had to be swapped out for “brew install --cask android-platform-tools”
After that I was able to run “sudo bash ./flash.sh” without any problems.