[P760] Bootloop, dead android and flash error

Hello,
About 2 months ago I had security error on my LG P760 device, so I flashed same ROM version (v20o) with R&D Test Tool. But there was sth wrong with the ROM and recently during reboot, android got into bootloop. I tried to hard reset it with the key combination, but there appeared the logo of dead android with red tiangle and so it did every time on.
I wanted to flash a new ROM with the same R&D Test Tool. Unfortunately (whichever official ROM version I wouldn't try) the upgrade process is stuck at 50% for a couple of seconds and then stops with an error. With a different flash tool the same happens. Here is the interesting part of a log from one of the sessions:

Code:

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16:29:22 : Step : TYPE_WPARAM_DOWNLOAD_FILENAME
16:29:22 : wParam : 2004,    lParam = 50
16:29:22 : MODEL DLL Event : (2004, 50)
16:29:23 : Step : TYPE_WPARAM_IFX_DN_POS
// UPGRADE IS STUCK HERE
16:29:54 : wParam : 2010,    lParam = 3017
16:29:54 : MODEL DLL Event : (2010, 3017)
16:29:57 : Step : TYPE_WPARAM_UPGRADE_ERROR
16:29:57 : CleanModelDll() :  Free Model.dll
16:29:57 : _DetachDLL Call
16:29:57 : _DetachDLL Call End
16:29:57 : Page_Error şÎşĐŔÔ´Ď´Ů
16:29:57 : ż©±â´Â Retry şÎşĐŔÔ´Ď´Ů

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Now, after few trials of upgrade that has been stuck at that 50%, device even doesn't show the boot logo. The only thing I can do with it is to enter S/W mode.
I ask for any help with it.