How do you have the UIPath installed and configured?
We faced similar issues down to me having it installed wrongly and I put this down to the earlier version of the installer which was not as obvious
As far as I know, to have a robot login via RDP or connect to a machine as if it was a user (i.e. not into an existing session) you must have the robot installed as a service
I guess if you have done this correctly, you could just check that there is a UIPath robot windows service installed and running
Good luck
Andy