I think that explanation by Autodesk is complete BS. They could VERY EASILY determine what is causing that error to be thrown when the drawing is being loaded. The problem obviously fires a trigger that displays that error message.
Also, once the drawing loads into memory, it corrupts something in the memory that is running the current C3D session. This causes the error to propagate to every drawing saved during that session. Because of this, I highly doubt it is a 'drawing setting'. I've never known drawing settings to be maintained in memory after the drawing has been closed. The only way to avoid it is to exit C3D before loading a 'good' drawing. WBLOCK is the accepted 'FIX' but it isn't really a fix, but a cheap workaround.
And it DOES sound like a virus, but it isn't the acad.vlx virus. But it behaves in a similar manner. Drawing files get larger, C3D runs slower, etc...
I tried creating a blank drawing and saving it as 'good.dwg'. Then, I loaded a corrupted drawing (to corrupt my current session), exited the corrupted drawing and then opened up the 'good.dwg' inorder to infect it. I then saved the drawing as 'corrupted.dwg'. The corrupted dwg is about 1 MB larger. I tried to do a hex compare, since I hadn't changed anything in the drawing, but you can't do it that way because the differences are too vast.
I'll figure this one out in due time. It is more of an annoyance than anything else and it just gives ammunition to people who are still ANTI-C3D at the office!