I've noticed a bug with RDP 6.1.7600. When I try to remote control another user on our Windows Server 2003 machine, I get an error that says "because of a protocol error, this session will be disconnected" and my RDP session is logged out. The error only happens when I try to control a Neoware Thin Clients. Is it because the RDP protocol versions are different?
I have seen this error when the client is set to use a visual style of some kind that the server has an issue with. What happens if you uncheck the "Themes" box on the RDP client config under the Experience tab.
I also have seen this error with network issues server side. Long time ago, but a driver update fixed them.