Make sure you are using Ram Concept 5.0.2. Older 5.0.0 and 5.0.1 commonly had issues importing AutoCAD files.
Also, make sure the CAD file is NOT OPEN when tying to import it into Concept.
Internal error importing dwg file
An Internal Error had occurred.
Object reference not set to an instance of an object. (Or similar)
Happens importing certain kinds of DXF/DWG polylines, unset text fonts, etc.