I came across this error in a Mixed 4.0 / 4.5 MPS Farm. The users would try to access the 4.5 server (which was the only 4.5 server in the farm) and they would receive this error message.
The complete error was:
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 http://someotherserver:80 [com.citrix.xml.NFuseProtocol.RequestAddress].
So what can we do about this error?