Clipboard empty but still get waring that clipboard is full

An Excel question:
Even though the clipboard is empty, I get an excel warning window that tells
me "Cannot empty the clipboard" when I click and drag data from one cell to
another. I have to click on the OK button in the warning window and then the
paste takes place. It doesn't stop me from doing it, it just adds an
additional and annoying step each time I want to move data within excel.
To add an element of confusion here, it does it in only some workbooks. And
if I copy data from one excel workbook where I have this problem to another
that does not, the problem is transferred to the receiving workbook.
If anyone can help, I'd sure appreciate it!!!!