i found my mistake! i constrained the slider and some other objects to an anchor object with mass 0. all objects that get connected to an object with mass 0 seem to freeze. and if other physics objects collide with the frozen constrained objects they freeze too! marco, is this a bug?

now i use a hinge constrained to the world for my anchor object.

@greyplasma: try to use bigger force values for your motors. i had to use values around 100000 to get it moving...