After my CNC stopped, measured a quick one, for deciding if the motor or the power supply is the guilty one.
It was the motor.
This means I've to face my largest fear regarding the CNC. The motor holder and the whole z-axis moving parts are manufactured from a single piece of metal. The motor was so tight in it that I cant remove it without much force (I tried already). I fear if I try to strain it to hard, it will broke.
Finally I was searching a wooden wedge in my workshop, removed the two screws, stave the wedge into the holder and with some force I was able to remove the motor.
Here is the guilty:
I ordered quickly a new one - this time a 400W one - from the eBay. Until it arrives I put the CNC aside. But it is possible that I tear down the motor to see what is the problem.