échec de rsync : Autorisation refusée (13)

J'essaie d'utiliser "rsync" pour copier le fichier sur une autre machine. Voici la commande que j'essaie:

sudo rsync -avztP --delete --recursive --inplace /folder1/folder2/folder3/myfile someuser@192.168.1.100:/backup/files -e "ssh -o StrictHostKeyChecking=no"

Je reçois cette erreur:

sending incremental file list
file1
    25728640 100%    15.38MB/s    0:00:02 (total: 100%) (xfer#1, to-check=0/1)
rsync: open "/folder1/folder2/folder3/myfile" failed: Permission denied (13)

sent 25736406 bytes  received 31 bytes  2258914.96 bytes/sec
total size is 25728640  speedup is 1.00
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1355) [sender=3.0.9]

Voici les permissions de la machine locale :

ls -ld / /folder1 /folder1/folder2/ /folder1/folder2/folder3/ /folder1/folder2/folder3/myfile

drwxr-xr-x  26 root  root      4096 Jul 25 23:00 /
drwxr-xr-x  25 root  root      4096 Sep 16 23:37 /folder1
drwxrwxrwx+ 12 root  root      4096 Feb 29  2020 /folder1/folder2/
drwxrwxrwx+  2 admin users     4096 May  4  2019 /folder1/folder2/folder3/
-rwxr-xr-x   1 admin users 25728640 May  4  2019 /folder1/folder2/folder3/myfile

Et les autorisations sur la machine distante :

ls -ld / /backup /backup/files/

drwxr-xr-x 20 root root  26 Sep  5 21:25 /
drwxr-xr-x  3 root users  3 Sep 13 02:07 /backup
drwxrwsr-x  2 root root   2 Sep 13 02:07 /backup/files/

Linux sur machine locale: Linux DiskStation 3.10.105

Linux sur une machine distante : Ubuntu 20.04.1 LTS, Linux 5.4.0-47 - generic x86_64.

Est-ce que je manque quelque chose?