Which version of AutoCAD MEP you are using? There was a known issue in AutoCAD MEP 2010 which was rectified in web update 2.
Do you have a 3rd party application like Quickpen, CADDUCT or EastCoast CAD? With a "Proxy Object" in the file, objects cannot be copied to the clipboard. The goes for Civil 3D objects wo the object enabler as well.
Thanks for the information.
It may be the reason you describe, but I'm not really sure what the file contains since it is a file produced by client and sent to us. I'll be more aware of this next time I recieve AutoCad files.
Had the same problem and ran across a quick work-around:
In my case I could use the ACAD COPY command (CO) but not the COPYCLIP command (CTRL-C)
I simply copied the whole drawing using COPY and moved off to the side. The resulting copy could be copied to the clip-board without any problem.
I'm using ACAD2013