I was trying to assist another admin with a login issue on a Windows Server 2008 terminal server when I encountered a slightly different login error than the one he was describing. When attempting to connect to the terminal server with a user not in the Domain Administrators security group I received the following message
“The requested session access is denied”
The problem, it turns out, was me. When connecting, I used a desktop shortcut for Remote Desktop Connect that had the “/admin” switch applied, which instructs Remote Desktop to connect to the Console session, which is restricted to administrators only. Using a regular shortcut without said switch solved the problem.