I'm accessing PLSQL Developer 9.0.6 via Citrix and have been encountering this. I thought it might have to do with which Clipboard is in use -- the local one, or the one on the Citrix host. Not sure if that plays into it.

This I found interesting:
* I opened a CLOB column and tried both Ctrl-C and the Copy button... in both cases I got the popup box and *nothing* was copied to the Clipboard.
* I think unlocked the result grid ("Edit data"), re-opened the CLOB and tried Ctrl-C and Copy button... again, in both cases I got the popup box, however, when I then did a Ctrl-V in a SQL window, it pasted from the Clipboard successfully.