Hello Andrea,

This is all about liveconnect - applet-to-javascript communication.
Applet configured to notify html page about its initialization, but sometimes fails:
17.11.2013 15:12:10 - DEBUG - jl_27937716_appletMessagePumper - Invoking JS method appletInitialized([Ljava.lang.Object;@82e4d6)
17.11.2013 15:12:10 - ERROR - jl_27937716_appletMessagePumper - JS method call failed: appletInitialized
17.11.2013 15:12:10 - ERROR - jl_27937716_appletMessagePumper - netscape.javascript.JSException: JavaScript error while calling "appletInitialized"
	at sun.plugin2.main.client.MessagePassingJSObject.newJSException(Unknown Source)
	at sun.plugin2.main.client.MessagePassingJSObject.waitForReply(Unknown Source)
	at sun.plugin2.main.client.MessagePassingJSObject.call(Unknown Source)
	at jmaster.jumploader.app.JumpLoaderApplet.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

I see following error in chrome developer console:
Uncaught TypeError: object HTMLAppletElement has no method 'getUploaderConfig'

Here is problematic function:

function appletInitialized(uploader){
	var applet = document.getElementById('jumpLoaderApplet');
	var origUrl = applet.getUploaderConfig().getUploadUrl();
	var currentFolder = parent.base64_encode(parent.ajaxplorer.getUserSelection().getContextNode().getPath());
	applet.getUploaderConfig().setUploadUrl(origUrl + "&dir=" + currentFolder + "&ajxp_sessid=" + clientId);
	applet.getUploaderConfig().setPartitionLength(partitionLength);
	applet.getUploaderConfig().setMaxFileLength(maxFileLength);
}

The solution is to use passed "uploader" variable (save it as global variable) for accessing exposed API insted of using DOM.
You can report this issue and solution to lacie