This through me for a look.  Seems that if you have a skype plugin installed, you cannot cut and paste formulas in Excel.  Pretty random till you think about it. The skype plugin is pretty aggressive trying to capture what it thinks are phone numbers and its totally possible the its intercepting the paste function and then gets confused when its a formula.  Fortunately, the fix is easy enough.

1. Open Control Panel.

2. Select Uninstall a Program under the Programs section.

3. Select Skype Click to Call, then choose Uninstall.

Once uninstalled, you may have to close and relaunch Excel. The problem pasting formulas should now be resolved.

Uninstalling Skype Click to Call

Credit Mitch Bartlett for this one.