Black Screen + Soft Reset + No Splash screen but phone works

Here again!

My mom's A20e LCD suddenly go to black. Black means with no light, no touch, nothing. (Note: LCD replaced 1 month ago, but don't get carried away by this information)

But it receive call (we can talk with BT), message sound and so on. In short, until just before the LCD, it works. And I feel like excluding that it's the battery because my mom is a big talker and she only charges phone in the evening.

The moment of death. During a call with me, suddenly, without the call ending, neither of them hears the other anymore. So I hang up the call, call back and... here we are.

Making a soft reset (10/20 sec Vol UP + Power) produce an intresting behavior, after 10 seconds makes a very fast and single flash. So no splash screen.

To make sure, I made a video and during this flash there is only light, no image at all. Clearly it's the LCD backlight and this is the only small instant that the screen "turns on".

Of course I don't have a LCD replacement to test, but I ask:

• Percentage chance it's the LCD?
• Percentage chance it's the Battery?
• Percentage chance it's the Firmware?
• Percentage chance it's the Kernel/Software?
• Percentage chance it's the MoBo?

Thanks a lot for your help