I’m having the same exception. Did you get a fix?
I probably won’t get to investigate further for a couple of days.
Here’s the exception with #### used to edit out private info:
[2020-12-07 22:43:42] EXCEPTION: lib/discourse.rb:92:in `exec': tar: /var/www/discourse/public/backups/default/XXXX-XXXX-2020-12-07-224327-v20201116132948.tar: Cannot open: Permission denied
tar: Error is not recoverable: exiting now
This private site is running on:
Discourse 2.7.0.beta1
Ubuntu 20.04
Digital Ocean