I used the script/flash files provided by galapagos on post #4.
It worked perfectly well. I was able to reboot, install superSU without loosing any data.
However, the device rebooted after some time. And now is stuck in LG logo.
I can go to "service mode" and repeat the flashing process through ADB:
adb reboot recovery
adb sideload root.zip
it flashes and askes for reboot. Then it is stuck again in LG logo...
Phone Model LGF240L from LG South Korea with stock ROM.
What can I do to recover the phone without loosing data?