Well, it depends on where the error is actually happening.
My guess is that for this error there’s a problem with the retention policy code such that it does the delete then either doesn’t record that it was deleted or records it then something else overwrites that (like it re-flags it as needing to be deleted).
So if the error happens again, you should be able to get the name of the missing file from the current run logs, then you can look at the previous run log and search for that file name.
You could also search the log for “DeleteRemoteFileset” or “RetentionPolicy” and you should see lines something like these:
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler:RetentionPolicy-StartCheck]: Start checking if backups can be removed",
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler:RetentionPolicy-FramesAndIntervals]: Time frames and intervals pairs: 7.00:00:00 / Keep all, 31.00:00:00 / 1.00:00:00, 365.00:00:00 / 7.00:00:00, Unlimited / 31.00:00:00",
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler:RetentionPolicy-BackupList]: Backups to consider: 02/11/2019 00:03:04, 02/04/2019 03:22:28, 01/31/2019 00:03:04, 01/19/2019 00:03:04, 01/14/2019 03:23:17, 01/07/2019 03:21:25, 12/30/2018 00:00:05, 12/23/2018 00:00:03, 12/15/2018 00:00:06, 12/05/2018 00:00:06, 11/26/2018 00:00:03, 11/14/2018 00:00:01, 11/03/2018 01:00:01, 10/26/2018 15:54:50, 10/16/2018 01:00:02, 10/08/2018 01:00:01, 09/28/2018 01:00:02, 09/21/2018 01:00:01, 09/14/2018 01:00:00, 09/07/2018 01:00:00, 08/31/2018 01:00:00, 08/23/2018 01:00:00, 08/15/2018 01:00:00, 08/01/2018 11:49:15, 07/23/2018 20:04:09, 07/01/2018 22:37:09, 06/17/2018 21:35:05",
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler:RetentionPolicy-BackupsToDelete]: Backups outside of all time frames and thus getting deleted: ",
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler:RetentionPolicy-AllBackupsToDelete]: All backups to delete: 01/19/2019 00:03:04",
"2019-02-19 00:08:25 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler-DeleteRemoteFileset]: Deleting 1 remote fileset(s) ...",
"2019-02-19 00:11:39 -06 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Delete - Started: duplicati-20190119T060304Z.dlist.zip.aes (882.28 KB)",
"2019-02-19 00:11:40 -06 - [Information-Duplicati.Library.Main.BasicResults-BackendEvent]: Backend event: Delete - Completed: duplicati-20190119T060304Z.dlist.zip.aes (882.28 KB)",
"2019-02-19 00:11:56 -06 - [Information-Duplicati.Library.Main.Operation.DeleteHandler-DeleteResults]: Deleted 1 remote fileset(s)",
"2019-02-19 00:12:52 -06 - [Information-Duplicati.Library.Main.Database.LocalDeleteDatabase-CompactReason]: Compacting not required",
In the above case, it’s the “Backend event: Delete - Started: duplicati-20190119T060304Z.dlist.zip.aes (882.28 KB)”, and following line that show a file was deleted.