Resolving "Remote Desktop Disconnected" or "Unable to Connect to Remote Desktop (Terminal Server)"

Solutions for errors such as “Unable to RDP,” “Remote Desktop Disconnected,” or “Unable to Connect to Remote Desktop (Terminal server)” are frequently asked for on TechNet Forums. There are 4 commonly seen Symptoms to this error:

1. You may be limited in the number of users who can connect simultaneously to a Remote Desktop session or Remote Desktop Services session
2. You may have a Port assignment conflict
3. You may have an incorrectly configured Authentication and Encryption setting
4. You may have a corrupted Certificate

The corresponding KB is:
Article ID: 186645 - Last Review: December 22, 2010 - Revision: 3.0
Troubleshooting RDP Client connection problems
http://support.microsoft.com/kb/186645

For more details check this blog post by the Remote Desktop Services (Terminal Services) Team:
http://blogs.msdn.com/b/rds/archive/2011/01/10/how-to-resolve-the-issue-remote-desktop-disconnected-or-unable-to-connect-to-remote-desktop-terminal-server.aspx