Have you checked the units of each of the parts of this assembly? Maybe WB needs for all the parts to have the same units.
This question is probably better suited for an Ansys Forum and not a SolidWorks Forum.
for the WB12.0 geomtry import options, try choosing only the types you wish to import. thus solid bodies or surface bodies. i believe that the mixed surface and solid cannot be performed within a single part. if you want to import surfaces and solids in the same WB model, then you need to create an assy of the solids and surfaces and then import the assy. line bodies cannot be imported from SW to WB. Line bodies only work for ProE.
The issue appears to be with Workbench v12 even though there are more options available regarding importing solids and surfaces.
There are no problems with same model in v11.
I finally got around to installing V12. I haven't done much in it yet, so I can't offer any particular insight into your problem. Did you contact your ANSYS VAR? Did you try suppressing parts in your assembly to see if a particular part or parts is causing the problem?