Backups failing, and admin page inaccessible

Since the most recent update, I’ve been getting error messages like this daily:

The backup has failed.

Here’s the log:

BackupRestore::BackupStore::StorageError
/var/www/discourse/lib/backup_restore/s3_backup_store.rb:63:in `rescue in unsorted_files'
/var/www/discourse/lib/backup_restore/s3_backup_store.rb:51:in `unsorted_files'
/var/www/discourse/lib/backup_restore/backup_store.rb:21:in `files'
/var/www/discourse/lib/backup_restore/backup_store.rb:26:in `latest_file'
/var/www/discourse/app/jobs/scheduled/schedule_backup.rb:11:in `execute'
/var/www/discourse/app/jobs/base.rb:137:in `block (2 levels) in perform'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rails_multisite-2.0.4/lib/rails_multisite/connection_management.rb:63:in `with_connection'
/var/www/discourse/app/jobs/base.rb:127:in `block in perform'
/var/www/discourse/app/jobs/base.rb:123:in `each'
/var/www/discourse/app/jobs/base.rb:123:in `perform'
/var/www/discourse/app/jobs/base.rb:185:in `perform'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/mini_scheduler-0.8.1/lib/mini_scheduler/manager.rb:81:in `process_queue'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/mini_scheduler-0.8.1/lib/mini_scheduler/manager.rb:29:in `block in initialize'

And if I try to go to the admin/backups page, I get an unsmiley face and:

Error while trying to load [/admin/backups.json](https://[forum]/admin/backups.json)

If I go to that URL, I get - while logged in as admin - “Access Denied”