Recently I cannot copy. Every time I select a text and Ctrl+C it, the process would hang and eventually crash. I even tried with a clipboard manager (qlipper, xfce4-clipman, clipit) but none of them worked.
Edit1: It happens when I try to copy something from a Source Game.
I've seen this a couple of times when Matlab or Octave are installed on Ubuntu.
Check if the Ctrl+C hotkey combination has been assigned to do another task.
Go to:
System
->Preferences
->Keyboard Shortcuts
Then scroll through the list and check if there's anything with Ctrl+C