Hi Bastian.
Sorry it took me so long to look into this.
This is caused by a bug in the vtkXMLRectilinearGridReader.
The reader actually repartition your data and do not keep the data the way it was partitioned. However it keep the vtkGhostType array the way it was, resulting in the issue.
This bug should be fixed, but, you actually do not need to provide the ghost cells, as they are computed on the fly when ParaView need them !
So my advice would be to just not generate GhostCells at all !