Servo command problem

Hi all, I am trying to set up a simulation where particles are inserted in an extreme of a rectangular pipe. Then, a movile surface should compress the particles (along the z axis). The sript (attached here) runs but few steps later the following error comes up:

ERROR: Mesh (id servo): Mesh elements have been lost / left the domain. Please use 'boundary m m m' or scale/translate/rotate the mesh or change its dynamics (../multi_node_mesh_parallel_I.h:680)

Indeed, when I visualize the case, four or five particles leave the domain.

At last I would like to ask how should I visualize the moving wall in paraview. I observed in an example case that people added the folowing command

dump dumpstl all mesh/stl 200 post/dump*.stl

but I even when the files are created I could not open them with paraview.

P.S.: I attached the case file and the log.