In Response To 'Re: Problem with copy and paste'
I have the same problem (I am on Linux) when I try to paste from one document to another, both of which were opened by separate Mathematica instances (i.e. double-clicking the files individually)
I have found a SIMPLE SOLUTION.
Just open the second document, (by going to File > Open) from the first document and then do the "copy as cell expression" and paste it into the second document!