UPDATE: This was fixed with the latest patch release 2018.1.3:
Error Message - Executor start process failed, reason System.Runtime.InteropServices.COMException (0x80070520): A specified logon session does not exist. It may already have been terminated.
We are aware that the latest Windows update(KB4088876 patch) caused failing to Start a Job from Orchestrator(if LoginToConsole parameter is False in the UiPath.settings file on Robot machine) and we are working on a fix that will be released as soon as possible.
I’ll keep you updated.