I've have similar situations. I figured it is usually connected to second language you have on the site. If there is no description (text to display for the item) in your language, then this will happen. It is easy to fix by going and adding your translation. Or if you deleted the original description and saved it like that, probably same thing will happen. And usually this missing key is a description that can be found in Pages or Forms apps for different blocks and such, and you can add it there.