Thanks Mark for your quick answer.
Your error message is due to the directory constant/polyMesh.
The region meshes are located in processorX/regionY/constant/polyMesh.
For some reason, if there is a constant/polyMesh directory (which happens everytime we run blockMesh to create a background mesh before decomposing and meshing in parallel), the reader looks for the equivalent directory in processors directories.
So it searches for processorX/constant/polyMesh and doesn’t look for processorX/regionY/constant/polyMesh.
Deleting the constant/polyMesh directory should solve the problem and get you to the issue I’m having.
EDIT:
I can’t use the reader module from OpenFOAM because I don’t have OpenFOAM on my desktop, the cases are computed on a remote cluster.
Since my cases are quite large, reconstruction is not really an option.