There are a number of reasons you can get a rate limiting response, e.g., due to the number of calls your app is making, or for other reasons such as other activity on the account or general Dropbox service issues.
In any case that you do get one of these though, the best practice is to retry the request, respecting the Retry-After header/'retry_after' value, if given, or using an exponential back-off, if not.
Can you have your app automatically retry the request after the specified number of seconds? Please let me know if that does or doesn't help.