The add-in is blank or isn't loading

If the add-in is blank or just loading, you could try clearing the browser cache on your computer, for Windows machines, you do this in Internet Explorer (Not Microsoft Edge)

Here a guide on how to clear the cache in Internet Explorer: https://clear-my-cache.com/windows/internet-explorer-11.html

If you're on a Mac, you clear the cache right inside the add-in in either PowerPoint or Word. Open the add-in, click on the "i" button that appears in the top right corner and select "Clear Web Cache" and then on "Reload". 
(If the "i" button isn't showing, click anywhere in the add-in and it will appear.)