Vitodens 300

Hi all
I have vitodens and hot water is working great.
The heat to radiator is not working, it works just if i shutdown the vitodens and start it again, then it pumps hot water to radiator in maybe 20 sec and it stops.

i can se that step motor is pushing up the pump when it stops, but if i take the linear step motor out it runs constantly and i have warm.