Pseudo-2D incomplete plane generation and ERROR des/check_cell_movement.f:178

Hi,
I was following the pseudo-2D DEM example in tutorial to build my own model to investigate the flowing particle behavior in a plane. As can be seen from the screenshot, my structure is a cluster of regularly-arranged spheres and particles can flow from the top and through the gaps. The generated plane (yellow region) shown in VTK is nonetheless incomplete and quite a lot of “fractures” can be seen. This further, as I think, leads to an error in the middle of run saying that " ERROR des/check_cell_movement.f:178 " .
I was wondering if this is a problem caused by coarse mesh? (My geometry size is X: 320 mm, Y: 320 mm, Z: 10 mm and cell size is X=Y=10 mm and Z=5 mm, particle diameter is 5 mm)
Does anyone have any idea to solve this problem? Thank you.
Xnip2020-05-05_20-00-07