Ultimaker 2 heatbed temp problem

[soul4u 0]

Hi, my Ultimaker 2 extended is suddenly not heating up the heatbed to the expected temperature.

The bed temperature stops randomly between 30° and 45° when it should go up to 80°. 

When I lower the bed temperature to the actual C° and heat up the nozzle, that the print can start, the bed heats up as normally did before and the wheel for manual choice of bed temperature does work again...

This is strange...anybody an idea, whats going on and how to fix it?

Thanks in advance,

BR

Thorsten