ServerIntermediary assumes that the client always sends a single listener:

case MBeanServerRequestMessage.ADD_NOTIFICATION_LISTENERS:
if (logger.traceOn()) {
logger.trace("handleRequest",
"Handle a ADD_NOTIFICATION_LISTENERS request.");
}

// Ideally, we should update this code to loop over the array.
// However, our implementation currently always sends a single
// ObjectName. The optimization that would send an array
// of them hasn't been implemented yet.
//

This assumption is obviously false when client and server don't come from
the same implemementation.

Also the returned type sent by our server and assumed by our client seems
to be Integer instead of Integer[], which means that our JMXMP client and
servers can only operate with each other.

(see ClientIntermediary and ServerIntermediary).