Hi All,
We have been facing this issue recently when the scheduled workflow tries to write the output to a .xlsx file. Configuration is set to Overwrite file(Remove) .
The interim solution is to rename the file and reschedule again.
This issue is cannot be replicated but occurs randomly after few workflow run after it is scheduled.
If any one has come across this issue - please let us know how you resolved it.

00:01:02.063 - Error - ToolId 156: An unknown error occurred in PI_Close.
Regards,
Alex