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Hmm that is interesting. I also had layer shifts in X (maybe around 2 - 4 times in a 4 hour print). After stepping down from 3600mm/min to 2400mm/min everything worked fine. So I didn't bother to look in to it any further. I blamed it on the igus bushings. What speeds did you use?

As far as I know those drivers step down power, when they don't have to move. Try moving the print head. If you can move it, it is likely that the drivers use less/no current (maybe coolstep: https://www.trinamic.com/technology/adv-technologies/coolstep/).

Can you try measuring VREF directly at Pin 17 (+ GND at Pin 18) of the driver?

Ah, I just found something! I guess you are right about the current.
In the Trinamic application note they used 0,18Ohms in the application example.
But in the table below they also list 0.15Ohms for 1.5A motors.
And the Anycubic steppers seem to be of that type according to this site: http://www.steppermotorshop.com/anycubic-nema-17-stepper-motor-bipolar-1-5a-40ncm57-1oz-in-4-leads-for-3d-printer-or-cnc-parts/
I totally missed that! :(
I guess I will order some resistors tomorrow ;)