URL Unauthorized access error while accessing files from dropbox?

Yes Error happens for all files with that name.

And this is the code we are using to retrieve the thumbnails of images.

public async Task<string> getThumbnailUrl(string path)
{
    string url = string.Empty;
    try
    {
        WebProvider objWebProvider = new WebProvider();

        var uri = DriveUtilities.GenerateSignature(new Uri(string.Format(AppConstants.DropBox_GetMediaItem, path), UriKind.RelativeOrAbsolute),
                      "GET", DropBox_AccessToken, DropBox_AccessTokenSecret);

        uri = await objWebProvider.GETRequest(uri).ConfigureAwait(false);
        if (!string.IsNullOrEmpty(uri) && !uri.Equals(AppConstants.Unauthorized))
        {
            DropBoxThumbnail result = JsonConvert.DeserializeObject<DropBoxThumbnail>(uri);

            if (result != null)
                url = result.url;
        }
        else
        {
            path = WebUtility.UrlEncode(path);
            var newuri = DriveUtilities.GenerateSignature(new Uri(string.Format(AppConstants.DropBox_GetMediaItem, path), UriKind.RelativeOrAbsolute),
                     "GET", DropBox_AccessToken, DropBox_AccessTokenSecret);
            newuri = await objWebProvider.GETRequest(newuri).ConfigureAwait(false);
            if (!string.IsNullOrEmpty(newuri) && !newuri.Equals(AppConstants.Unauthorized))
            {
                DropBoxThumbnail result = JsonConvert.DeserializeObject<DropBoxThumbnail>(newuri);

                if (result != null)
                    url = result.url;
            }
            else
            {

            }
        }
    }
    catch { url = path; }
    return url;
}