If you are getting the 403 Forbidden error when connecting to Amazon S3 storage check if your access key ID has permission to list the available buckets.
In case your access key ID is restricted to a single bucket you need to enter it as default remote directory on the
advanced tab of the site entry in the Site Manager. Make sure it is prefixed with a slash.
Also double check the entered credentials. Watch out for character case and leading or trailing whitespaces.