URL Unauthorized access error while accessing files from dropbox?

I'm not sure what's going wrong in your code, so I thought I'd instead provide some working code I wrote for you. The method below returns the full URI to download a thumbnail from Dropbox.

Note that I'm not using OAuthBase.GenerateSignature. That method returns a URL-encoded version of the signature, which I found hard to work with. Because we're using PLAINTEXT signing, it's very easy to construct the signature yourself. It's just the app secret, then an ampersand (&), and then the token secret.

static Uri GetThumbnailUri(string path, string appKey, string appSecret, string token, string tokenSecret) {
    var oauthBase = new OAuth.OAuthBase();

    var ub = new UriBuilder(@"https://api-content.dropbox.com/1/thumbnails/auto/" + path.TrimStart('/'));

    // get an empty HttpValueCollection
    var parameters = HttpUtility.ParseQueryString(string.Empty);

    // add all the OAuth parameters
    parameters["oauth_consumer_key"] = appKey;
    parameters["oauth_token"] = token;
    parameters["oauth_nonce"] = oauthBase.GenerateNonce();
    parameters["oauth_timestamp"] = oauthBase.GenerateTimeStamp();
    parameters["oauth_version"] = "1.0";
    parameters["oauth_signature_method"] = "PLAINTEXT";
    parameters["oauth_signature"] = appSecret + "&" + tokenSecret;

    // set the query string for the URL
    ub.Query = parameters.ToString();

    return ub.Uri;
}