It was the time for getting out my trusted multimeter and some tools.
The display and control panel was still working as normal.
I removed the plastic panel (kept in place by 8 Philips screws) covering the motor and control circuit board.
The first I set out to do was to test if the motor was still working. I disconnected the black (-) and red (+) wire from the circuit board.
I connected my multimeter directly to the motor.
With the multimeter set to Ohm it should read 0 Ohm or close to 0. It did in my case so I went on to the next test.
With the multimeter set to VDC, I rotated the motor quickly by hand. If the motor is working it should give some voltage reading from the display of the multimeter. It did in my case so I went on to the next test.
I connected the motor directly to a car battery. The motor was turning. My conclusion was then, that the motor was working as it should and the controller circuit board had to be the problem.
I removed the controller from the treadmill by undoing 4 Philips screws (red circles) ,removing the 2 wires for the motor (light blue circles) and 2 power wires (green circles).
At the rear of the controller circuit board I found a broken leg on one of the components (red circle). Because of the weight of the component and the fact that it was not fixed to the circuit board I am sure that vibrations from running on the treadmill had caused the fault.
Before soldering it back on I glued it to the circuit board using a glue gun (melted glue). Hopefully that component will not make any more trouble in the future.
Some more info:
Belt type: 483J (483 mm)