Could you post a screenshot from Repetier (or whatever software you use) to show us what the object should look like?
Looks like your Y-Axis is literally losing all the steps in one direction (and some in the other)
To make sure it’s not a mechanical problem, check the screw in the pulley of that motor.
After that I’d measure the reference voltage on the stepper driver (should be around 0.9V).
If that’s correct switch driver boards between X- and Y-Axis to see whether the error travels with the board and after that wires.