When a Windows XP client attempts to connect to a Windows 2000 Terminal Services server, the connection fails and they receive:
Because of a security error, the client could not connect to the terminal server. After making sure that you are logged on to the network, try connecting to the server again.
Microsoft made a change to the certificates used by the Microsoft Clearinghouse to authorize Terminal Services Licensing servers.
To fix the problem:
1. Use the Registry Editor to navigate to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\TermService\Parameters key on the Terminal Services server.
2. Delete the following Value Names:
X 509 Certificate
X 509 Certificate ID
3. Close the Registry Editor and restart the Terminal Services server.
4. Use the Registry Editor to navigate to the HKEY_LOCAL_MACHINE\Software\Microsoft\MSLicensing key on the Windows XP client.
5. Delete the MSLicensing key.
6. Close the Registry Editor.
7. Connect to the Terminal Services Server. If the problem is NOT resolved, call the Microsoft Clearinghouse, at 1-888-571-2048, to deactivate and reactivate the Terminal Services Licensing server.
NOTE: Since Windows XP has a built-in CAL for Terminal Services, deleting the MSLicensing key has no effect on license allocation in your organization.