Dell Printer Keypad repair (C1765nfw)

[XPModder]

Hello,
I have a Dell C1765nfw multifunction color laser printer. This printer is working perfectly fine with the exception of the buttons, which do not work at all.
The printer does not have a soft power button, but rather a good old power switch, which is not part of the keypad/display assembly and obviously works fine.
I got the printer for free as 'not working' as the display was not working correctly a little while ago. I already repaired the display problem (found a cheap replacement display, as the COG controller of the original was faulty) and also replaced the ribbon cable from the main PCB to the keypad/display pcb with a new one, as the old one was delaminating... (yes, the replacement is the exact same pin pitch and count and is fully seated at both ends.)
After fixing the display and flashing the newest firmware version, I found out that the keypad is not working. The printer does not respond to any button presses at all, no matter what button(s) I press.
I have now also replaced a 74HC244 and a 74HC174 on the keypad pcb, as I suspected one or both of them to be bad, but that did not fix the problem either. I did measure the voltages on the supply pins and they seem fine. The 5V rail is slightly high, but still easily within tolerances.
I checked all resistors and diodes, as well as checking for diode drops across the transistors, both on the keypad board and on the main board next to the corresponding connector and they are all fine.
There is also no visible damage to any of the boards or components.
As I was unable to find any good documentation on this printer, I do not have the schematics and have not (yet) gone down the rabbit hole of reverse-engineering the entire board.

The photos are all of the keypad/display pcb. The orange ribbon goes off to the LCD and the white one goes to the main board of the printer. Its a one-sided board, so nothing on the top, apart from some leds, buttons and a connector for a buzzer...

Any ideas what the problem could be?
I hope its not something broken with the application processor on the main board, but I dont think it is, since everything else works just fine...