I'm just using openDocument, without overriding it.
Yes, the files are on the internal boot volume (APFS).
The error was flagged in the UI. TM reported that the backup hadn't completed, and in System Settings it reported the name of the file.
It has done this to various files: the only connection is they are all PDFs that are or have been open in my app. Closing the app and running TM fixes the error.
As usual, I can't find any relevant logs in Console.
Topic:
App & System Services
SubTopic:
Core OS
Tags: