The error code shown is -196608 (0xFFFD000 in hex). From what I can gather, this indicates a problem with permissions or authentication when the script is run as SYSTEM. See: https://www.briantist.com/errors/scheduled-task-powershell-0xfffd0000/
What exactly does the WindowsUnquote.ps1 do? Do you have commands in the script that depend on a PATH environment variable? Does it access a shared folder on another server? Does it need to access restricted folders on the current server?
Another possibility is that your command needs double-quotes around the script file parameter: "C:\temp\WindowsUnquote\WindowsUnquote.ps1". Looking around at old forum posts, most of the time I see double-quotes being used.