Question Y axis stops on D840 performance kit.

Hello,
I have added a performance kit to my STEPCRAFT2 D840.
It has been working without any issues for several years, but suddenly I encountered a problem with the Y-axis.
Specifically, when I perform a reference operation right after startup or manually move the Y-axis, the machine stops with an error when the Y-axis has moved approximately 4.2mm.
Even if I press the emergency stop button and reset the device, the same issue persists.
I want to identify the problematic area and repair it.
Therefore, I would appreciate it if you could provide me with the steps to investigate and identify the cause of the problem in order to fix it.
Thank you.

The cause was found.
I solved it by removing the sensor of the rotary magnetic encoder.
I think it's a sensor for Y-axis synchronization, but it works even if it's removed.
Is there any problem if I leave it as it is?