I fixed the problem myself. The ibmjceprovider.jar of IBM's jdk seems to be signed incorrectly. That's why the classloader fails to load any RSA-related classes (needed for SSL-communication). The workaround is to rebuild ibmjceprovider.jar (resulting in an unsigned jar that will afterwards be loaded without any problems). The exception thrown when trying to load the classes seems to be caught without logging any messages (that's why I couldn't find any hints in the logs).