Old HP thin clients cannot connect to Windows 2008 R2 RDS server

I fear that you may be having a problem with "licensing". I saw something similar with a couple Wyse thin clients but rather than troubleshoot the issue the Customer just opted to replace the hardware (because they only had two of the devices).

This long thread on social.technet.microsoft.com is filled with users having a similar experience. The users there seem to have tied the problems to activating the license server. Their devices worked properly before the grace period expired.

There's a disheartening statement in that thread: "The official word from Microsoft is 2008R2 RDS does not support clients below 6.0. Microsoft support did not offer any further assistance other than to go to the thin client vendor." That's pretty disappointing if it's true.

There's also a proposed solution that involves deleting some registry values from under the "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\RCM" registry key that some reported success with. I'm pasting it here, slightly edited, for reference, but the credit goes to EricWy:

• Open Remote Desktop Licensing Manager
• Right-click your Licensing Server name select Properties.
• Change Connection Method to 'Web Browser'
• Go back to the Licensing Server, right click your server. Select `Advanced / Reactivate Server'
• Reactive server via the given Wizard + web browser
• Delete the following registry values below the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\RCM (the values will be re-created automatically when you reboot):Certificate, X509 Certificate, X509 Certificate ID, X509 Certificate2
• Reboot