If it moves fine in z everywhere else, it sounds like it could be some mechanical issue. Like the toolhead hitting something in that corner, or something not being assembled quite right.
#Qgl and movement speed
safe fable
Can it move the toolhead to (0,0) without it binding skipping?
Where are your qgl probe ponts (corners)?
safe fable
Could you post a klippy log (ideally one that includes a qgl attempt that fails)?
safe fable
Hm. I haven't used cartographer or eddy current bed scanning things myself so not really familiar with what kind of issues that could be related to. But if everything feels "mechanically sound" (moving Z up and down with the toolhead at the "weird corner" I'd maybe try swapping that stepper driver for one of the other Z motors and see if the problem moves with that (or at least rule the stepper driver out as a cause).
When it's malfunctioning, do the other three z motors move as they should and it's just the one at one corner that isn't moving (but just "buzzing in place")?
safe fable
Aha, I didn't get that, I thought the issue was with Z movement.
Anyway, if your gantry is moving nicely (and feels equal in all directions) when you move it by hand, there's probably something wrong with one of the A/B motors (the one that runs it in the front left <-> rear right direction, I always forget which one it is). Could also be something like improper belt tension, or a belt that isn't routed right.