The Citrix servers reported that they are too busy to provide access to the selected resource. This message was reported from the XML Service at address [com.citrix.xml.NFuseProtocol.RequestAddress].
Got this issue the other day on my web interface. I was unable to launch any app from any server in my farm. Qfarm / load showed normal load on all servers.
Issue turned out to be a corrupted load evaluator. Switching to default load evaluator fixed the issue. I was then able to recreate my custom load evaluator and reapply it.