On FreeNAS ZFS system if the share or the disk of totaly full you can not delete any files or you can not move them between folders ….. you must free disk space since ZFS pools should never be filled to capacity.
dd if=/dev/null of='file path'
The path to the backup files is “ /mnt/your_pool/the_files” then
rm 'file path'
Once some space will be recovered everything will start working again.