screwy encoder input (5i25 + 7i77)

My mill has been running flawlessly for a couple years. Yesterday it started intermittently throwing a following error on the X axis. This morning, it was no longer intermittent but happens every time I try to enable the machine.

When enabled, the x axis slowly starts moving and then throws the error. I realized that while the physical joint is moving, the position isn't updating. However, if I move the Y axis, BOTH the X and Y position changes. I switched connectors on TB3 encoder0 and encoder1 to verify that the X axis encoder is still sending pulses (it is).

I'm not using TB4 at all so I suppose I could move the X and Y there but I'm wondering about root causes. How should I proceed?

I could be something like a broken encoder ground wire but it

could also be a bad 5i25/7I77 or loose/bad cable

( if the encoder mux pin from the 5I25 to the 7I77 fails for some reason,
encoder0 will mimic encoder1, encoder 2 will mimic encoder 3 etc)

Strange. Both ends of the cable are tight with thumbscrews but I pulled off both and reseated. Pulled out the 5i25 and reseated. At least for now, seems to be working. Strange. Thank you.