On the configuration pages, on the Backup tab, on the Advanced Backup sub-tab, in the Backup File field set, there is a File Encryption setting that references the Drupal AES encryption module. That module is:
A Drupal module that has not been ported to Backdrop
Unsupported due to a security issue the maintainer didn’t fix
The first thing that probably should be done is to remove that option or re-word it so it is not pointing to an unsupported Drupal module.
On the configuration pages, on the Backup tab, on the Advanced Backup sub-tab, in the Backup File field set, there is a File Encryption setting that references the Drupal AES encryption module. That module is:
The first thing that probably should be done is to remove that option or re-word it so it is not pointing to an unsupported Drupal module.
The next thing that should be done is to port the Encrypt module and use that one, since support for it was added to the 7.x-3.x branch of the D7 module in March of 2019: https://www.drupal.org/project/backup_migrate/issues/2135813