I have the same problem with dwg/dxf from Rhino to Artcam 2008 (build151). What is more when the message appears nothing else can be done and I have to kill the Artcam process manually. But I found a turnaround : I import the dxf/dwg file in Adobe Illustrator and then I save it in EPS format. Don't try to save in native Illustrator format (.ai) even if Artcam can read it because this method will also produce an error in Artcam. Once the EPS file is generated just import it in Artcam... and it works.
Hope it helps