When I got this error, it was version 0, the latest. Your original post was version 3. I think Duplicati checks all the versions, and if so, I wonder if errors in old versions might be introduced by operations that change them, such as compact? It’s easy to tell if compact has run by looking in the job logs. Here’s an example:
CompactResults:
DeletedFileCount: 14
DownloadedFileCount: 5
UploadedFileCount: 4
DeletedFileSize: 195118534
DownloadedFileSize: 183452545
UploadedFileSize: 96832127
Dryrun: False
MainOperation: Compact
ParsedResult: Success
Sometimes reporting options such as email also give log details you wouldn’t ordinarily see without setting up a --log-file (which would be a reasonable idea because it may give some clue as to what’s going on…).
Things like logs and log analysis are a great start. From a debug point of view, a system without network issues, sleeps, or reboots should be a good candidate for looking at what other sorts of things make this.
If your external drive has space to spare (at least you’re not paying a cloud provider by amount stored…), –no-auto-compact would remove that from the picture (to see if it helps). You can also try letting versions build up instead of trimming with retention policy. Basically, experiment to see what things avoid the issue.
If you add a log file to the job or server, --log-file-log-level=information would have compact and delete info.