Sounds as if you might try turning OFF object detect and command invoking
and then load the drawing in question. If the error persist, this might suggest
that you need to open a Land drawing and attach to the C3D drawing and
query everything back into the Land of Land.
If turning off Object Detect/Command Invoke works let us know.
And or send me a sample of the afflicted file to dissect in the lab.
Can we be sure that they are the same version, 2006, 2005?