One of the two Y-stepper motors stops stepping

[Pulsar07]

When working with small CNC mill based on a MKS DLC32 V2.1 with MKS A4988 v2.0 drivers and two Y-stepper motors (17HS8401S with 1.5A rated current), I saw that one of the y-stepper motors stopped working for seconds. After aborting the job immediately and moving the X- and Y-axis with normal positioning commands, both Y-stepper motors work fine again. Starting new mill jobs work fine, but sometimes the left stepper motor stops working again !?

How are the two Y stepper motors plugs on the board connected ? Serial or parallel ? On my configuration all 3 motor drivers have the same ref voltage settings of 0.95V. Is this too less for the Y-axis? and therefore the reason for loosing steps or stopping one motor at all?

What can be the reason for this behavior?