I tend to look for the simpler explanations first.
If your keyboard was previously working,
the first place I would check is the MMJ
on the back of the terminal.
I've seen everything from malformed pins,
to corrosion, to spider webs, to dead bugs
inside RJ12 / RJ45 connectors.
Another point of failure is the?MMJ on the keyboard cable;
you can try removing that, and re-crimping it.
T