This could be caused by some add-in.
You could try disabling all add-ins to see if that solves the problem. If it does, you can try enabling them again, one at a time, until you find the culprit.
To disable/enable add ins:
-Go to “Tools” menu, click “Trust Center”, and then click “Add-ins”.
-Select the first Add-in type in the “Manage” box and then click “Go”.
-Select or clear the check box for the Add-in that you want enable/disable and then click “OK”
-Go to the next Add-in type, and perform the same actions.