In addition to the other suggestions, if you’re doing this because there’s no great GUI for it, you might also consider using GUI Commandline option for the job, which takes care of this, with or without a passphrase.
Command edit is required to change The BACKUP command into The PURGE-BROKEN-FILES command, however it’s probably a matter of preference whether it’s nicer to edit here, or to build the line from scratch unassisted, or to use the job’s GUI Export as Command-line to get the command-and-options to edit some.
Doing it in the GUI Commandline probably also has the advantage (?) of keeping the server tied up so that scheduled runs of the same backup can’t happen at the same time you’re independently fixing the backup.
Thanks everyone. I chose to just pause backups indefinitely in the GUI, and then run the CLI command with the --no-encryption flag.
I tried the GUI commandline option, but it wasn’t working (probably because I had a bunch of flags from the starting backup command that didn’t make sense in context).
Now that I’ve purged the busted files, things are backing up again. Fortunately all my source data is still there, so I shouldn’t lose anything.