Usually with unattended, the robot will log in to create the session without anyone else (human) logging in to that session. Once the robot finishes a process, it will end the login session.
It sounds like you are trying to log in to the session manually, then run the robot? You can do this, but you won’t want to minimize or disconnect while the robot is running or it will stop it (though, you can suppress the minimized problem potentially with this: https://robot.uipath.com/docs/executing-tasks-in-a-minimized-rdp-window
However, going back to my first point, the unattended robot will login to the session with no human involvement and should work. If it is not working, something is going on with the server, like a popup message or the Win 8 / Win Server Start up Screen is showing upon logging in. - This is difficult to identify from my perspective because you would need to see what the server is doing when you “manually” login. I would first suggest Signing out the robot user id from the session (by using Ctrl+Alt+End > Sign Off), then log in and see if anything shows up that could get in the way of the Ui interaction.
Another possible way to identify an issue is use the Take Screenshot and look at what the screen looks like when the error occurs (surround workflow with Try/Catch so you can take the screenshot when the error occurs.)
So finding these details may help with solving it.
For more direct support, I would recommend escalating the issue to help desk: https://www.uipath.com/company/contact-us/contact-technical-support
Hope this helps some!