expired_access_token

An 'expired_access_token' indicates that the API call failed because it was made with a short-lived access token that has expired. Please note that Dropbox issues short-lived access tokens which automatically expire after a few hours.

Apps can 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. Refresh tokens do not expire automatically and can be used repeatedly. 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.

You mentioned you're using the official Dropbox API v2 Java SDK though. On Android, that would actually handle this for you automatically, as long as you use the authorization flow functionality provided by the SDK. (You should also upgrade to the latest version of the SDK, currently v7.0.0.) As long as you use the provided authorization functionality, starting with the startOAuth2PKCE method, it will handle the refresh process for you. You can find an example Android app here. For instance, you can see where that starts in the code here and how the result is handled here. When you use that functionality as shown, the SDK will catch those 'expired_access_token' errors for you automatically and get a new access token using the refresh token when needed.