Sony Xperia SP- Soft-bricked, can't get PC to recognize

Lent my Sony Xperia SP (C5306) to someone, it came back soft-bricked. I assume this, because the phone vibrates 3 times when charging now, and the screen will not ever turn on at all. Sometimes the RED led at the bottom lights up or blinks twice, but never any other color.

I tried installing various older & newer versions of Sony's "PC Companion", to get the drivers working, but there's no hope there. Stupid Sony insists on updating it or it won't work, and the update always fails on every...

Sony Xperia SP- Soft-bricked, can't get PC to recognize

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