I got the korean version of the (almost) Optimus G (Optimus LTE2, f160k) and ran into the same security error problem that happened out of the blue.
The steps that I undertook below resolved the problem for me and has been proven to work on Optimus G by fellow members.
1) Download the latest KDZ from LG's server for your phone via the app below (kudos to the creator)
2) Make sure you have LG United Drivers already installed, otherwise download from below
3) Download LG KDZ Updater
4) Download the manual adb wipe batch file
5) Put your Optimus G into Download Mode by switching it off, attaching the usb cable to the phone, holding both the up and down volume buttons at the same time and lastly inserting the usb cable into the computer; wait for the drivers to finish loading if doing it for the first time.
6) Unzip the downloaded updater zip file and run KDZ_FW_UPD.exe. Use the settings below
Phone Mode: CS_Emergency
KDZ file: navigate to the folder which you have saved the downloaded KDZ from the step (1)
Click Launch Software Update and wait for the flash process to finish
7) Your phone will restart itself when IParam = 90 is displayed and proceed slower until you see IParam=1 again (which signals the end of flash, you can safely exit the program then and remove the cable from the phone). If phone boots up normally, congrats, skip the steps below and proceed to use the phone as per normal.
However, this most probably will not be the case for newer Optimus-es, as the data partition somehow wasn't wiped even when KDZ-ed, remaining 'dirty', thus causing bootloop or security infringement.
8) If the phone is stuck at the booting up LG/Telco logo, you will need to manually wipe the data partition.
9) Force the phone off by holding the power button for appx 15secs.
10) Put the phone into recovery mode by holding the volume up and home button concurrently, then press the power button. Let go of all the buttons after you see the LG logo on screen for the 2nd time and the screen goes blank thereafter (backlight may still be on). The phone will kind of reboot and then enters recovery mode. Repeat the process if you did not get it the first time.
11) With the phone in Recovery Mode, connect it to the computer, wait for the drivers to load (if doing it for the first time), unzip the wipe batch file downloaded earlier from step (4), double click on wipe.bat to execute the wipe command.
12) Your phone will auto reboot when wiped and (hopefully) boot as per normal.
Hope with this simple guide fellow members will feel safer when rooting or removing non essential LG wares from their Optimus G.