Just so I understand, at render time, 3ds Max gives an error "render elements error output" ?
So sounds like that is where the issue is, rather than the LightMix being in error. If I understand the error correctly (a screengrab of the error may help confirm it is what I am thinking you mean) And that sounds like the 3ds Max problem where it has invalid paths set for render elements.
Deleting and recreating the render elements is one solution for when Max does that, does that resolve the issue in this particular scene?
Thanks!
Tom