I just ran across this problem today and I have been looking on this site for a fix and found that none worked. But this is what I ended up doing to fix the problem that I had.
Excel closes and restarts. I found that if you go to the “More Command” on the top toolbar and click on the Add-Ins then go to the bottom drop down menu to the “Disabled Items” and click on the items that are disabled to enable, then close Excel and re-open Excel it should work.
This fix will probably work for Word, PowerPoint, Outlook .
Also if the Add-Ins can be unchecked then uncheck them, if you have to go to Regedit then change the numbers to 0