Microsoft released a new Kb article related to being unable to remove a RD Session Host or RD Virtualization Host from a RDS 2012 deployment.
“…Assume that you try to remove a Remote Desktop Session Host (RD Session Host) or Remote Desktop Virtualization Host (RD Virtualization Host) server from your Remote Desktop Services (RDS) deployment. If the target server was already deleted from Active Directory, you may receive the following error message:
Could not remove the configuration. Object reference not set to an instance of an object.
This issue occurs because the removal operation tries to contact the server in order to remove the Connection Broker from the RDS Management Servers internal group. The operation fails because the computer object cannot be resolved in Active Directory…”
“…To safely remove the server from your RDS deployment, contact Microsoft Customer Support Services. For a complete list of Microsoft Customer Support Services telephone numbers and information about support costs, visit the following Microsoft website: Contact Us (http://support.microsoft.com/contactus/?ws=support)
Important We do not recommend that you manually edit the database that is used by the RDS deployment…”