[LG Optimus 4X HD] Aborrajado:
Same problem here, I´ve tried to downgrade with the same results.

Using KDZ or UpTest I get the following:

09:06:59 : Start fn_StartUpgrade

09:06:59 : Extract kdz file

09:07:05 : kdz decrypt Success

09:07:08 : Extract file Success.

09:07:08 : LGMobileDL Load.

09:07:08 : Port = -1

09:07:08 : Connection check start.

09:07:22 : Check Phone mode = 2

09:07:22 : Phone type check.......

09:07:24 : LGMobileDL.DLL Loading....

09:07:24 : _SetAuthMark Fail

09:07:24 : _SetAuthMark Ok : wAuthMark(65535)

09:07:24 : _DetachDLL Call

09:07:24 : _DetachDLL Call End

09:07:24 : Param : Path = C:\ProgramData\LGMOBILEAX\Phone\LGP880AT-00-V20a-234-000-MAR-26-2013+0.dz

09:07:24 : Param : moduleDir =

09:07:24 : Param : waitTime = -1970404774

09:07:24 : Param : UsbHighSpeed = 1

09:07:24 : Param : PhoneMode = 2

09:07:24 : Param : BinVersion = V20A_00(1)

09:07:24 : Param : AuthMark = 65535

09:07:24 : Call fn_StartUpgrade

09:07:24 : 3GQCT : wParam = 2005, lParam = 0

09:07:24 : 3GQCT : wParam = 2005, lParam = 4

09:07:24 : 3GQCT : wParam = 2003, lParam = 5

09:07:24 : 3GQCT : wParam = 2005, lParam = 4

09:07:34 : 3GQCT : wParam = 2005, lParam = 0

09:07:34 : 3GQCT : wParam = 2005, lParam = 1

09:07:34 : 3GQCT : wParam = 2003, lParam = 3015

09:07:34 : 3GQCT : wParam = 2003, lParam = 1

09:07:34 : 3GQCT : wParam = 2005, lParam = 1

09:07:34 : 3GQCT : wParam = 2003, lParam = 1

09:07:34 : 3GQCT : wParam = 2005, lParam = 1

09:07:34 : 3GQCT : wParam = 2005, lParam = 2

09:07:34 : 3GQCT : wParam = 2005, lParam = 3

09:07:34 : 3GQCT : wParam = 2003, lParam = 3

09:07:34 : 3GQCT : wParam = 2005, lParam = 3

09:07:34 : 3GQCT : wParam = 2005, lParam = 4

09:07:34 : 3GQCT : wParam = 2005, lParam = 5

09:07:34 : 3GQCT : wParam = 2007, lParam = 0

09:07:34 : 3GQCT : wParam = 2005, lParam = 5

09:07:34 : 3GQCT : wParam = 2003, lParam = 201

09:07:34 : 3GQCT : wParam = 2005, lParam = 5

09:07:35 : 3GQCT : wParam = 2003, lParam = 401

09:07:35 : 3GQCT : wParam = 2005, lParam = 5

09:07:35 : 3GQCT : wParam = 2003, lParam = 200

09:07:35 : 3GQCT : wParam = 2005, lParam = 5

09:07:35 : 3GQCT : wParam = 2003, lParam = 202

09:07:35 : 3GQCT : wParam = 2005, lParam = 5

09:07:35 : 3GQCT : wParam = 2003, lParam = 203

09:07:35 : 3GQCT : wParam = 2005, lParam = 5

09:07:38 : 3GQCT : wParam = 2005, lParam = 10

09:07:38 : 3GQCT : wParam = 2003, lParam = 204

09:07:38 : 3GQCT : wParam = 2005, lParam = 10

09:07:52 : 3GQCT : wParam = 2003, lParam = 205

09:07:52 : 3GQCT : wParam = 2005, lParam = 10

09:07:52 : 3GQCT : wParam = 2003, lParam = 206

09:07:52 : 3GQCT : wParam = 2005, lParam = 10

09:07:52 : 3GQCT : wParam = 2003, lParam = 207

09:07:52 : 3GQCT : wParam = 2005, lParam = 10

09:07:52 : 3GQCT : wParam = 2010, lParam = 207

Then the phone gets stuck and it never reboots, using LGFlashTool port 4 it keeps resetting over and over again, I thought it was the USB cable but reading what you wrote its probably not the problem, I suggest you look for a folder called efs where is supposed to be the IMEI of your phone, if you have it you might be able to do some sort of recovery, Ive found some guides on google but unfortunately I dont have the folder and didnt back it up, anyways, if you find out whats happened to our phone let us know. Thanks

---------- Post added at 11:05 PM ---------- Previous post was at 10:35 PM ----------

Forgot to add, I flashed kdzs with no problems at all about a year ago, then I unlocked the bootloader and started to flash roms in the following order: Zaiben ZRC3, werewolf, Vanyr, and finally stayed with Zaiben ZRC3 where one day just couldnt get any signal and the imei appeared as null.

Ive tried this youtube/watch?v=rPHdI0bYf2U, installing differents roms and going back to stock but I cant manage to restore my imei without the efs folder.

My phone is brand new not a dev one, any help will be appreciated.

Thanks