The JClouds library is unable to communicate with S3 compatible services running on non-standard ports. This has been traced to the v4 authentication code.
In HTTP, the host header can "optionally" include a port number. In practice, this is included when the HTTP service is running on a non-standard port (i.e. not 80 or 443). The AWSv4 auth code uses only the hostname from the URI for the host header, as can be seen at:
This is fine for AWS's official S3 endpoints which are only accessible via TCP ports 80 or 443, but is incorrect for communicating with "compatible" endpoints running on non-standard ports. In the non-standard case, the host header should include a :port suffix.
See HTTP RFC: https://tools.ietf.org/html/rfc7230#section-5.4