Dropbox is failing to reply to the app

A user of my app is reporting a new problem with Dropbox. The permission screen appears and then he gets an error:

Most of the users who have tried out my app with the TMS FNC Cloud Pack library are reporting no issues with Dropbox.

Can you please make sure you are using a valid callback URL?
Are you using a port number?

Example:
http://127.0.0.1:8000