Should you receive the following error message when opening a template;
The reason this error message is thrown is due to the credentials no longer being stored in the add-in or the add-in not being able to automatically obtain the credentials from SharePoint.
All that needs to be done to resolve this, is to sign into the Doc/Slide Generation add-in of the opened template.
Should it keep happening, it could be your organisations GPO which is forcing this to happen or network connectivity issues.
For more information on signing into the product add-in, click here