Xgen batch render turn randomly grey



I’ve been strugling with an horrible issue with Xgen, renderman and maya 2017
I think video are more talkative than words :



I’ve got no idea where this could comme from…
I’m using Xgen with Maya 2017 and Renderman 21.5
These are feathers archives, with Renderman shader applied on it
maya and xgen path are set correctly
patches for batch rendering are exported

Any ideas?