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It was the grub screws on the pinon gears on both Y motor shafts. One was really loose and one was a bit loose!

At least I got to spend ages checking motors and drivers.

Did you learn any more about this error?
The 2 steppers I use for my Y axis have lost a lot of holding torque.
The fault was intermittent for a while and now permanent.
Nema 23 4Nm Bipolar Series...