Here the solution:
1) Download a battery stats wipe app from Play Store (requires root privilegies!)
2) Full charge battery and wipe
3) Full discharge the battery
4) Full charge battery and wipe
5) Disinstall all Lg software and drivers
6) Clean pc and registry with ccleaner
7) Change User on Windows with another with administrator privilegies
8) Follow this guide: http://lg.storagecow.eu/?page_id=12
It resolves two problems:
1) "Battery level is low and upgrade is unavailable"
2) Fix B2CAppSetup problem: "To check for software upgrades. you need to connect your PC to the internet and press "Continue"".
These are all the steps that I've followed for flashing kdz and restoring stock.