Possible reasons for following symptom?

you still miss the most important pointy what power supply are you using what are the volts and current rating( amps)
of the power supply

if it is moving forward fine and stalls going backward , this is most likely mechanical binding
however if you have the settings of the steppers wrong this will exagerate the problem even more

As DaBit mentions change to 15000ns and 30000ns and leave it at that , if it is still problematic then solve the binding issues , try removing the steppers and turn the shaft by hand , it should turn smoothly and equaly in both directions ..
if it is harder to move one way than the other , then you have mechanical binding issues .

also do not have the stepper drives on a high step rate , take them to half step , the higher the step rate the lower the torque you have available , this is a trade off with using steppers , if your power supply is marginal, this will only increase the problems your experiencing ..

so one step at a time , and you should find the problem .

Folks, thank you all!

setting the jumper on the driver card for the X-Axis to 1/8 did the magic. Now the X-Axis moves without stalling. So I will start further tests on the upcoming weekend.

Regards,
Michael