JChannel.open() fails because the properties of the previously created channel cannot be fetched correctly. If set store the setup_string, and someone changes the instance variables, e.g. via a setter (UDP.setMulticastPort()), then open() will use the old props !

open() used to call Channel.getProperties(), but since we cannot simply return the old 'props' value (mostly not used anymore !), we have to either generate the properties from the instance vars annotated with @Property, or somehow store a 'configuration' of the properties in the channel.
Maybe a ChannelConfiguration class ?