Only the most recent failed backup ever seems to be available to restore, but there is always very little material contained within it. Therein lies a clue: I had moved my server directory away from a too-small partition a while back, as a possible solution to problems I was encountering, and the only contents of these ephemeral failed backups were a couple of files from the Duplicati server directory. As it turns out, I’d foolishly moved the server directory to a partition which was getting backed up. Backing up the 9GB backup database, with all its churn (and perhaps also file locks?) might be the problem. Retrying with revised configuration now.
Given that the consensus elsewhere seems to be against including the server directory in backups, this might be worth warning about/protecting against?
Will report back on progress of retry.
EDIT: Retry #1 appears to have produced a complete error-free backup…