If you are trying to open an AutoCAD DXF file from the late 80s or 90s and get things like "Unknown error" in Illustrator or other software, just get a blank page in AutoCAD or a generic "failed" message, try this.
Open the DXF file in your favorite text editor.
Near the top, maybe line 6 or so, find $ACADVER
.
Under that, you should see 1
, and then the DXF drawing database version number.
If you see something outside of the official list (http://images.autodesk.com/adsk/files/autocad_2012_pdf_dxf-reference_enu.pdf), it probably won't work.