Because of a security error, the client could not connect to the remote computer

Because of a security error, the client could not connect to the remote computer. Verify that you are logged onto the network and then try connecting again.

Recently I was presented with the above error when attempting to remote desktop to a Windows Server 2008 machine on a client VPN from a Windows Server 2003 virtual machine. The error message suggested a problem with my VPN authentication or connectivity to the remote machine. However after a bit of digging I realised that this was caused by the remote desktop settings on the Windows 2008 machine. In particular the setting related to the version of Remote Desktop that is required. When remote desktop is enabled an administrator has to choose one of the following options:

• Allow connections from computers running any version of Remote Desktop (less secure).
• Allow connections only from computers running Remote Desktop with Network Level Authentication (more secure).

More info here: http://technet.microsoft.com/en-us/library/cc794832(v=ws.10).aspx

If the administrator selects the latter then you can't connect via a Windows Server 2003 machine unless you update your Terminal Services client to version 6.1. There isn't an official 6.1 release of the Terminal Services client for Win2k3 however there is a security update available for SP2 that does update you from version 6.0 to version 6.1.

To check which version of Terminal Services client you have installed go to C:\Windows\System32\ and check the properties of mstsc.exe (since updating to 6.1 on Win2k3 I have version 6.0.6001.18564).

You can download version 6.0 here:

http://www.microsoft.com/en-us/download/details.aspx?id=21296

Download the security update to upgrade to version 6.1 here:

http://www.microsoft.com/en-us/download/details.aspx?displaylang=en&id=22926

Credit to this article for the explanation of how to apply the 6.1 update:

http://technotes.khitrenovich.com/install-rdp-6-1-windows-server-2003/