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.
Solution : run Windows Update on the Windows client computer, it should be enough to solve the problem.
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.