Access Token not found error on delta call after authorization

You're using Dropbox API v1, a.k.a. the Core API. API v1 supports both OAuth 1 and OAuth 2. You seem to be mixing OAuth 1 and OAuth 2, but you're only supposed to use one or the other for any particular API call.

I.e., the "access_token" parameter and the "Authorization" header are ways to supply an OAuth 2 access token. The other "oauth_*" parameters are used for OAuth 1.

You should only use one of these methods at a time. We recommend using the Authorization header to pass an OAuth 2 access token. If you use that, you can get rid of the URL parameters.