'Failed to render garbage matte' help!

Working on a project masking clothes and hands in Motor v1.1.2, I get a ‘failed to render garbage matte’ error message halfway through the render. It seems to be linked to a particular layer.
Could it be the mtion blur?
What is this due to and what can I do to get around the problem?
Thank you.

This sounds like a bug. Please post the .motor project file either here on the forum or by email to support AT imagineersystems.com, along with a description of how to reproduce the problem, and we will investigate.