Remote Desktop Services is currently busy

If you ever see this error when a user is trying to log on to a Remote Desktop Server:

The task you are trying to do can't be completed because Remote Desktop Services is currently busy. Please try again in a few minutes. Other users should still be able to log on.

Log on as an Admin
Load Remote Desktop Services Manager
Switch to “Processes” tab (the user won’t be listed in the Users or Sessions tabs)
End any processes listed for that user

This is usually a Group Policy process getting stuck

Once the processes have ended, the user will be able to log on fine.