This document will explain the Event History Code Unknown Error (0xffffffff) and provide a possible fix for it.
You open the View > Event History and see 'Unknown Error (0xffffffff)' under 'Core engine/definitions
This code indicates there isn't enough disk space to download Core Engines/Definitions
The error in ST.ServiceHost.managed.log:
TaskHandler.cs:126|Failed to execute 'ST.Services.Deployment.Synchronization.DataFilesDownload.RunAsync'.: System.AggregateException: One or more errors occurred. ---> System.ComponentModel.Win32Exception: Unknown error (0xffffffff)
Clear up drive space on C:\