The used MPI suffers from a known defect where unweighted topology
communicators cannot be duplicated with MPI_Comm_dup.

This issue was reported to MPICH as issue #5628, see

<https://github.com/pmodels/mpich/issues/5628>

for more details.

Consider rebuilding MPI with the patch in
config/checkpatch/mpich_3.x_unweighted.patch
