I am running a Nonlinear study and I have three parts moving and making contact with each other.  During the sliding of one component over another I get an error, 'improper contact surfaces for a contact area'.  Does anyone know what it means and how to solve it.  I have tried different mesh densities, contact types (surface to surface, node to surface), and changing from fixed to automatic time steps, etc.  No difference.  My materials are set at Plasticity - von-mises.  I have also tried playing with increasing the Tangent Modulus, but to no effect.

Any help greatly appreciated.

Thanks

Rob