It's not really acting out, but I didn't know how else to describe it
Anyway, I
1)charged it for about one hour
2)changed the SD card
3)reflashed the firmware that @DNSB kindly provided
4)installed KOReader on it to see if it'd work, and it did.
The only problem are those lines on the upper part of the screen (which were already there before I reflashed the firmware).
The top area is still responsive, by the way; when I click the buttons on KOReader and stock OS, they still do what is expected of them.
So, any ideas of where should I go from here? I suppose I can still do some margin-fu on KR to skip that upper area, but that would kill a third of my read estate