Now the Monitor applet is failing to load in the browser. There's no error message in the Tools->Error Console and the Java console doesn't load, presumably because no applet is loaded. At this stage I have no idea what the problem is. It works in Applet Viewer, so maybe the signing is faulty. Running any applet in the current browser gives the message that it is simply ignoring applets.
Apparently the reason was that I had openjdk AND sun6 jdk installed. So I had two incompatible versions of java. So I guess the plugin called the wrong java and failed to load the applet.
Although it took ages to load on the testbed (probably because of all the machines it had to detect) it eventually came up and monitored the network.