I can’t help with Windows, since I haven’t one. And Windows is known to be specially problematic for
fastboot. Although you can test running a manual Fairphone OS or Fairphone Open installation.
I can help with Xubuntu not recognizing your device, though. I guess you are running Xubuntu 16.04 LTS, right? 16.04’s
fastboot packages have a bug, and you need to manually install the
android-sdk-platform-tools-common package with:
$ sudo apt-get install android-sdk-platform-tools-common
$ adb devices and
$ fastboot devices should output your device correctly without
sudo. If that’s not the case, reboot or run
$ sudo service udev reload and try again.
After configuring this, you could follow the instructions to restore Fairphone OS/Open above in the thread and re-launch the Ubuntu Touch installation through their installer.