I used TrueConvert and saved back to ver 14 then opened in ACAD2000.
It will not allow any copy & paste within the DWG.
Also opened in ACAD2006, stil no Copy & Paste.
The Copy showed 1 object found bur no paste,
I noticed thet there is a DXF code 370 in most objects, even when I add a LINE the code is there.
(370 . -1)
This in not in my other drawings & something is adding it within this drawing.
That's all I have time for..
Oh, I used MP's (c:getproxies and it generated an error.