SharePoint AddIn - 401 - Unauthorized: Access is denied due to invalid credentials.

I am in need of an experienced SharePoint developer to help troubleshoot a 401 - Unauthorized error we recently encountered when accessing our newly created SharePoint AddIn. We have already tried using different user credentials to access the AddIn, so we know that is not an issue. We are certain this is a new AddIn, and we are using Windows authentication for this. We would like someone who has experience with SharePoint AddIn (Apps) and this type of error to help us quickly identify and resolve the issue.