hmmph…
This machine is pretty frustrating.
Now one of the motor drivers is faulty.
Easy to identify when swapping driver plus the check voltage is too low.
I like building stuff. But this just keep delivering fault after fault.
Well it seems to be a price to pay when your buying a kit. On an already built printer some factory checks are performed which is impossible with a kit.
I have a defect driver too. If you quickly check it, it seems to be OK but is not actually (voltage drop and overheating until it stops working).
Ask for a replacement (support at velleman dot be).
In the meantime you can fine tune the axis and the extruder like I did, swapping the good ones.
And OK, I’m sure you’re hopping up and down with impatience like me 
Have you checked the reference voltage for the drivers?
They should be set but mistakes happen.
Also make sure the fan on the bottom is running and is blowing the right way.