Backup failed with permission denied

I’ve been using Discourse successfully for a while now, but backups have all of a sudden stopped working.

Relevant line from the log is below. Not sure what would have caused this permission error to start happening from out of the blue. I was going to start mucking around, but thought I’d ask here if there was an easy known fix.

...
 Making sure archive does not already exist...
[2019-04-10 03:44:45] Creating empty archive...
[2019-04-10 03:44:45] EXCEPTION: /var/www/discourse/lib/backup_restore/backuper.rb:225:in `create_archive': 
tar: /var/www/discourse/public/backups/default/xxxx-forum-2019-04-10-034440-v20190306154335.tar: 
Cannot open: Permission denied
tar: Error is not recoverable: exiting now