This is a solution for anyone that has encountered this problem: Backup .htaccess File Error.
Having recently upgraded a Wordpress solution and migrating it to another server, I encountered a similar error where the /backup/.htaccess file is not able to be reached. Having spent a few minutes researching it, I found that the “Backup Wordpress” plugin, that the client had added to their Wordpress solution, was the culprit. The plugin was looking for the .htaccess in the directory path of the old server, and not the new one.
Instead of trying to dig through PHP files to try and find some kind of absolute path, I simply deactivated and deleted the plugin, upgraded to the latest version of Wordpress, and added another “Wordpress Backup” solution via the built-in plugin search in the admin area.