Had the same problem on a used 5” (S-505) purchased in the hope of transplanting its screen to my dead unit.
Alas the problem was with one of the COG ICs not the controller.
Someone did suggest years later fusing the IC back on by spot heating via blue laser (GOGGLES!!!!) where the interconnects go, on the basis that the fail can be one of the contact areas on here rather than the ribbon cable.
The idea being that the spot size was smaller than the actual silver copper based micro links so fusing where the bad area was might achieve some level of function.
Incidentally the S-600 also has this “feature” which is why they don’t make these any more.
Additionally they are prone to failed clock crystal also causing garbled screen, USB not working etc.