While working on the problem that IBB is choosen, even if Bytestream is possible, i discovered that the PrivacyManager in Spark creates a new DiscoveryManager to check if privacy is supported.
This new ServiceDiscoveryManager will, in addition to the standard Manager, reply on all disco#info requests. This results in the issue, that disco#info requests are replied twice. In some circumstances this will cause that Bytestream fails and IBB is choosen.
I created a patch for Spark to make the PrivacyManager use the standard DiscoveryManager instead creating a new one.