Ok .. I must admit, this one has me stumped ... I'll give the scenario and you can test it on your installation to see if the result is the same...
Scenario:
VBA Automation to open DXF file and back up the version to R12
User selects file via Common Dialog (found elsewhere on this forum)
VBA directs AutoCAD to open the DXF file and make it the active drawing
VBA attempts a saveas to a previous version and the error happens...
Sub DXF2R12()
Set DXFDoc = Application.Documents.Open ("C:\MYFILE.DXF")
DXFDoc.Saveas "C:\MYFILE.DXF", acR12_dxf
End Sub
Anyway, there is no problem if saving "dwg to dwg", "dwg to dxf" or "dxf to dwg" only "dxf to dxf" fails ...
In the meantime I have resolved the issue by using this workaround ...
Sub DXF2R12()
Set DXFDoc = Application.Documents.Open ("C:\MYFILE.DXF")
DXFDoc.Saveas Environ("TEMP") & "\MYFILE.DWG, acR15_dwg
DXFDoc.Saveas "C:\MYFILE.DXF, acR12_dxf
Kill Environ("TEMP") & "\MYFILE.DWG"
End Sub