Question : Unable to RDP to Windows XP SP3 from RDP client 6.x yet RDP 5.x works perfectly

I have a Windows XP SP3 PC with remote desktop enabled running in a domain environment. I am connecting remotely to this PC via a VPN connection. If I use RDP client 5.x  to connect everything works perfectly however if I use RDP client 6.x it fails to connect, I don't even get the login screen appear.

To complicate matters if I take a laptop with RDP 6.x installed on to the local site and plug it into the LAN I can connect perfectly.

Any help is much appreciated.

Answer : Unable to RDP to Windows XP SP3 from RDP client 6.x yet RDP 5.x works perfectly

This problem turned out to be an issue with an Astaro firewall. Once the firewall had the firmware updated the problem was resolved.