There are some weird bugs going on and so far there is no sure answer as to why. Try restarting...if that doesn't work try making a guest account in System Preferences>Accounts click the + on the bottom (may have to click the little padlock bottom left and unlock it if you have it locked) to add a new account. Log into the new user account and see if copy paste works there. When you do log into the new account remember in that account your preferences like tap to click etc won't be on so you will have to actually click the touchpad button until you change them, and the dock may look different etc but dont worry when you go back to your original account the settings will be as you had them. If it does switch back to your original account and see if it now works. I know that sounds odd but it has solved at least temporarily a couple small things in some cases.
Hopefully this stuff is just a bug and an update will fix it.