Backup downloads and restores are counted in user quota
In our situation a user has a 1GB quota. They have used 700MB, and they backup tarball is 400MB. In the panel they attempt to download a backup. The backup file is copied to their home directory, and the chown is attempted as per https://git.ispconfig.org/ispconfig/ispconfig3/-/blob/3.1.15p2/server/plugins-available/backup_plugin.inc.php#L88 . The chown fails because it would exceed the user quota. As a result, the file cannot be downloaded by the user because it is not owned by them, and the permissions are 600 (which is correct).
It seems it would be good to somehow exclude these backup files from the user quotas. Does a feature like that already exist, and if not, what would be a good solution?