A coworker of mine just had the same problem. What she was trying to copy to the clipboard was a block definition. I went into the block to look for AEC objects, and none were found.
I ran the AUDIT and PURGE commands, and everything was clear
It turns out the block (a civil plan) had other blocks nested within. I went into the main block through the block editor, exploded the nested blocks, and saved the main block
I ran the AUDIT and PURGE commands again, and this time it found a whole whack of errors.
Once they were clear, I was able to copy the objects to the clipboard
Example of Civil AEC objects:
With plain autocad polylines are polylines, but in Civil, polylines can be convert to AEC Contours, they hold locations, elevations so when you make a cross section trough them, the program generates hills and valleys and calculates cut / fill of how much dirt, etc......
I just got this bug today for the very fisrt time (in AutoCAD 2012).
I tried to Copy/Paste a BlockReference (for a title block filled with attributes). So I went in Block Editor to check if I could Copy all the entities that way and it worked. So in my case the problem was not the BlockDefenition so I closed the Block Editor.Then I did a "ATTSYNC" command to reset the faulty BlockReference. And after that the Copy/Paste did not failed!
Just happend to me - a rare problem. I've tried purging and recovering the drawing without success. Then saved to R12 DXF, closed it, opend the DXF, saved to current 2013 DWG and the problem went. After correcting some settings and defining new layouts the model id fine.