Error messages

An 'expired_access_token' error indicates that the call failed because it was made with a short-lived access token that is no longer valid because it has expired. You'd need to get a new short-lived access token when that occurs.

Note that Dropbox is no longer offering the option for creating new long-lived access tokens. Dropbox is now issuing short-lived access tokens (and optional refresh tokens) instead of long-lived access tokens. You can find more information on this migration here.

Apps can still get long-term access by requesting "offline" access though, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example.

Note that this is something that would need to be implemented by the programmer of the app, so if you are not the programmer responsible for this integration, you may need to get an update from them to support this.