Hello, your advice saved one VTH1550CH form brick.
However, I noticed I cannot downgrade it to FW version: General_VTH151X_Eng_P_V4.000.0000.0.R.20180622, it will not flash properly.
I succeeded to flash version: General_VTH151X_Eng_SIP_V4.300.0000000.8.R.20190316
Now I am trying to flash one...