Hi, I have an app that allows the users to connect their Dropbox accounts and see the content directly in the app. When the user enters his credentials and grants permission in the Dbx form, my app receives the refresh token and I use it to programmatically get a new short-lived access token whenever it's needed. This worked since some weeks ago and then, without the users or the app doing anything, the refresh token and expiration date returned null. If the user revokes the connection and reconnects, the app receives a new access token but the expiration date and refresh token are still null.
What are the scenarios where the Dropbox service returns a null refresh token?
What can I do now to get a valid one?