While trying to connect to a Terminal Server only a grey screen is displayed. No other error messages occur.
The most common cause for this sort of behavior occurs when the terminal server is unable to obtain a license.
Restart the terminal server or validate the license server
This error means that the account you're using is missing a requirement to be able to use Remote Desktop.
The account you try to log in with must have the following:
Be a member of the Administrators OR Remote Desktop Users group
Have the "Allow Login through Terminal Services" user right.