I don’t think a backup was interrupted; nothing in the job log anyway. Here’s the full text of the warning:
Duplicati.Library.Main.Operation.Backup.UploadSyntheticFilelist-MissingTemporaryFilelist]: Expected there to be a temporary fileset for synthetic filelist (70, duplicati-i9c3da6fcb08a4a89b804d98f8e2724af.dindex.zip.aes), but none was found?
But there is now a problem as a a verify via the GUI gives an error message “Detected non-empty blocksets with no associated blocks!”, and when I attempt a repair it finishes successfully far too quickly; and then validation fails with the same error. Now attempts to run the backup fail with the same error message.
I will recreate the database and see what happens.