OMG!!! it's outlook.
OMG!!! It's not Outlook! It. Is. Excel. The fact that closing Outlook worked for you (for now) does not mean that there is not a problem
In. Excel.
There is no pressure on Microsoft to fix Excel as long as people keep coming in here crying "Yipee, I found a (probably temporary, until the
next program that Excel doesn't work with) solution to my problem! I shut down some other program to "fix" it".
As Deron Braun put it with exceptional clarity:
The problem is Excel. Sure, one application or another may be using Clipboard but the common denominator is still Excel. Blame all the applications and forms of usage that bring about this error and still it is Excel.