This may not be helpful, but
I once had the experience of being unable to paste and it being greyed out when I was using a utility to manipulate the contents of the clipboard.
When you copy points or polylines, it seems that text data is stored in the clipboard. When you paste into a text editor, a very long piece of text appears.
Because I was using a utility to manipulate it (I wanted to add "¥" to a file path in Explorer), the text data that should have ended with </ARCGIS> ended up as </ARCGIS>¥, and it seemed that it was not recognized as data to be pasted in ARCGIS.