You probably enabled daily backups and the system tries to store them on Amazon S3, but gets an “Access Denied” error. First thing you could do is switching to local backup storage by going to /admin/site_settings/category/all_results?filter=backup%20location and changing the setting to “Local”.
If you want to store the backups on S3, please make sure that the S3 credentials are still valid. I recommend following the Configure automatic backups for Discourse guide.
And yes, I know, the error handling at /admin/backups could be improved. I’ll work on that…