A customer came to me the other day with this problem. I checked updates and yes , the latest office 2016 updates were installed.
I checked system updates and yes latest El Capitan 10.11.6 installed. We tried change all sorts of preferences, killing pref file but no success.
Finaly i decided to uninstal the whole Ofice 2016. Info about that here.
After reinstalling and running the updates and input the accounts IT WORKED! I have no idea what caused it though.