Solving "Model has errors that need to be corrected in.."

I am trying to get GrabCAD to do a differential infill project: one end of the part can be a sparse infill, while the other end needs to be solid for maximum support. My understanding is that I need to make a multi-body solid or an assembly and use the Advanced FDM feature of GrabCAD in order to select different bodies to choose different fill styles, as described in this tutorial: Variable Density FDM parts with Advanced FDM in GrabCAD Print

My problem is that when I import the assembly (or multi-body solid), one of the bodies has a persistent error:

"Model has errors that need to be corrected in the originating CAD system"

There's no indication as to what's causing this. The part doesn't need to be regenerated or rebuilt in Solidworks, there's no errors anywhere except the one above.

What's frustrating is the basic FDM functionality of GrabCAD can easily fix this with it's integrated "heal" option, but this option is not available for the Advanced FDM?

Any help here?