You are using the Internet Explorer converter. The line in the log "Failed to to query user token. Last Error: 2" is from the service attempting to start the IE converter engine and failing because it is failing to query the user token, cannot find it. When you installed WebGrabber enter a user in the WebGrabber IE Logon window that opened. Here is a link to the guide for setting up the IE converter.
When that is done the WebGrabber user is logged in and disconnected, allowing the IE converter to run as that user and you will see the WebGrabber user logged in in the task manager.