This is usually because the Group Policy forbids that user or group from logging in.
run "gpmc" (not "gpedit.msc" as that is for local computer settings when you are not using Active Directory) or go to Administrative Tools and Group Policy Management.
Edit the default domain policy like below........
Start -> Administrative Tools ->Server Manager
Under "Computer Information" click "Configure Remote Desktop".
Under "System Properties" choose the type of "Allow connections" you want.
That's all there is to enabling RDP Access in 2008 server.........
I keep getting messages like this shortly after using the proxy (it works for a few seconds/page loads and then stops):
channel 12: open failed: administratively prohibited: open failed
I'm not sure what the issue is unless there's some kind of hardware firewall on the other end. I've used this exact configuration on multiple servers with no issue and even disabled iptables etc..........