I don’t think this is well-understood (old issue came back), but the lead developer gave a comment here.
If you have Windows or Linux LVM (maybe in a recognized config…), you could try –snapshot-policy=on.
2.1. Rollback Journals explains the naming and use, and I’d guess it would be locked, which probably is causing the “Failed to process path”, but the detailed cause is on the line after the one in the summary. Emailing yourself a backup report, or setting up a –log-file will catch more error information, if you want.
If snapshots are not possible, not backing up that area would be another workaround. If you really want backup of the database (some people do, e.g. to avoid Recreate), see the later posts at top link above.