I've read lots of blogs on this subject going back to 2005!!
I'm sure that Paul Simon is right (see his submission to this thread) and it's a question of finding which program on your machine is locking the clipboard. I do not run the programs listed in various solutons suggested (eg on Microsoft website) nor am I in a networked or virtual environment so for me those aren't the locking programs (but might be for you). Similarly I don't have the RDP task going in my processes. For me the locking program is the Skype Add-in.
I am not a sophisticated user and am scared of altering my registry so didn't want to go there.
I have now been able to reproduce accurately the "cannot clear the clipboard" message by turning on and off the skype addins in internet explorer. This is easy for amateurs to do and might be one of the more common clipboard locking programs:
I first confirmed that I can turn on/off the problem in Excel by opening/closing internet explorer.
Then I disabled the skype addins:
Internet Explorer: Tools menu --> Internet Options ; Programs Tab ; Manage Add-ons button; Toolbars and Extensions selected in panel on left - scroll down to find skype add ons. Press Disable button.
NB have to restart Internet explorer before this works.
.... 4 days later.... it's still working