TBH, Jolla isn’t responsible of Sony devices (well, Android devices in general actually) being delicate, when trying to get them flashed. The unfortunate part is that flashing must be “difficult” so that it cannot be performed by accident.
It’s also true that some OSes and USB controllers just don’t play ball nicely 
Here is a few things to try:
- Make sure the cable is good: boot the phone to Android and make sure you can e.g. move a few photos
- Use another USB port. May seem stupid, but it affects things… Don’t ask me why.
- Run the commands as root (
sudo) - Disable USB power save settings using
powertop(see this post/topic) - Use older
fastboot(see link above) (I hope X10II really doesn’t need this…)
After I had my X10II finally unlocked, I had to flash it twice, because it went into reboot loop. No errors, but it just wouldn’t boot up… After second try, no errors either, it worked.