I am tryng to use amazon S3 SDK to upload images on RIAK, but i am constantly receiving 403 forbidden. Supplied credentials are good. Any ideas?

Here's the code:

Amazon.S3.AmazonS3Config config = new Amazon.S3.AmazonS3Config();
config.ServiceURL = "http://s3.mydomain.hr";
Amazon.S3.AmazonS3Client client = new Amazon.S3.AmazonS3Client("*********", "**************", config);
Amazon.S3.Model.PutObjectRequest por = new Amazon.S3.Model.PutObjectRequest();
por.BucketName = "mybucket";
por.CannedACL = Amazon.S3.S3CannedACL.PublicRead;
por.Key = key;
por.InputStream = f;
Amazon.S3.Model.PutObjectResponse resp = await client.PutObjectAsync(por);