Finally I found a workaround which doesn't work for the currently used OpenORB but for JacORB (3.4).
Oracle obviously updated the release notes suggesting to configure only the org.omg.CORBA.ORBClass
property but to ommit org.omg.CORBA.ORBSingletonClass
which means the default singleton implementation shipped with Java will be used which is visible for the SystemClassLoader.
This may not work with all ORBs and in fact it doesn't work with OpenORB as it checks org.omg.CORBA.ORBSingletonClass
but my application works without problems with the mentioned version of JacORB now.