Keypad keys malfunction on Nokia X6 may also possibly caused by a faulty keypad flex, cleaning or by replacing it will help solved the problem.
If after replacing and ensuring that the flex is okay but the keys still unresponsive, you may then consider that the problem lies on a faulty keypad circuit's hardware components on the PCB board. This solution below will tell us which components holds and controls the keypad keys to function or work.
From the keypad pin connector there are three line filter coils located near to it, have a check on this components and replace if found damaged.
From the keypad filter coils the lines is feeds to a keypad filtering chip or keypad IC, you may then check this chip, trace its line paths and replace if found damaged.
The I/0 Expander Chip is the keypad controller components, which holds and control the keypads including the Touch Control Module. You may also rework or replace this chip for further harder keypad problem repair on Nokia X6.