K8400 Z-axis not moving

Hi guys!

I’ve had my printer for about 2 months, yesterday I woke up to a messy print. After some troubleshooting it seemed like the Z-Axis at some point during the print stopped moving. So I cleaned everything and tried to move the Z-Axis Manually through the “Move Axis” Setting on the printer. I can hear the step motor spinning but the axel is not turning.

So i dissasembled the rod from the step-motor and unmounted the motor from the printer. I re-attached the cable just to test the motor by itself. It seemed like the motor didn’t have enough power to turn the axel. So i thought it was a faulty stepmotor.

Exchanged the stepmotor for a new one through warranty. Got the new motor today and replaced it, but the problem still remains. So from this point on i have no clue what to do.
So I am turning to you guys for help.

Is there an issue with the flexible coupler?

Did you check the stepper driver adjustment?

Did you swap the stepper driver with one from e.g. X,Y or extruder?

Do you mean this part?

Yes I replaced it with a new one but the problem excists.

Which stepper driver do i need to replace ?