This error message indicate that you are trying to connect from an Windows XP computer to a Windows Vista/Seven or 2008 TSPlus server. It happens because Windows XP uses RDP 5 protocol while Windows Vista/Seven/2008 uses RDP 6.

Solutions : The use of Windows Update on the Windows XP client computer should be enough to solve the problem. You can also look to update only the RDP 6 client for Windows XP here : <<link>>

If you cannot update your Windows XP client computer, it is also possible to manually correct this issue by following this procedure :

Open a registry editor (regedit) on the client computer and find this location :

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\TermService\Parameters.

 
Within the "parameters" key, locate all Certificate and X509 Certificate and delete them. Reboot your computer. The certificate keys are rebuilt once XP is restarted and now compatible with Vista/Seven and 2008.