I’ve just had the misfortune of trying to export a very basic model with Arnold Shaders from Maya as a .ma file. Upon trying this I get the fateful error.
A solution I’ve very quickly found is this python script from Fred Lewis (http://www.movingmedia.com/python-scripts-maya/)
# Unlock unknown types and delete them import maya.cmds as mc import sys unknowns = mc.ls(type='unknown') if unknowns: mc.lockNode(unknowns, lock=0) mc.delete(unknowns) unknowns = mc.ls(type='unknown') if not unknowns: sys.stdout.write('All unknown nodes have been deleted.') else: sys.stdout.write('Somehow, not all unknown nodes were deleted.') else: sys.stdout.write('There are no unknown nodes in the scene.')