SSH remote copy always failed, falling back to sshfs copy

[Dummyzam]

I'm trying to solve an issue from some times with setting up a sshfs backup using rsync to my nas running TrueNas.
Each time, SSH remote copy fail and, as expected, it fallback to sshfs copy... which take ages.
I've red @jadudm recents posts (and @james , both very useful, thxs !) which seems to be very close to my issue, with an close setup.
I've tried everything i could base on it but i'm probably missing something and i keep having this issue :

• initial upload (or diff upload after first backup) is done without issues (in the "snapshot" folder if i understood correctly)
• but when it try to do a remote copy it always fail so, the backup process upload every thing again...

Here is a log extract example (192.168.0.10 being my TrueNas ip) :

2026-04-01T02:01:21.275Z shell: filesystem: ssh -o "StrictHostKeyChecking no" -i /tmp/identity_file_dba91bb5-5fb9-46b8-91cc-69d69437aaea -p 22 myuser@192.168.0.10 cp -aRl snapshot/app_6da85c9a-fbf0-4563-baaa-1ad3080cf467 2026-04-01-020004-530/app_next.mydomain.com_v5.7.0
(node:210013) [DEP0190] DeprecationWarning: Passing args to a child process with shell option true can lead to security vulnerabilities, as the arguments are not escaped, only concatenated.
(Use `node --trace-deprecation ...` to show where the warning was created)
2026-04-01T02:01:21.582Z shell: filesystem: ssh -o "StrictHostKeyChecking no" -i /tmp/identity_file_dba91bb5-5fb9-46b8-91cc-69d69437aaea -p 22 myuser@192.168.0.10 cp -aRl snapshot/app_6da85c9a-fbf0-4563-baaa-1ad3080cf467 2026-04-01-020004-530/app_next.mydomain.com_v5.7.0 errored BoxError: ssh exited with code 1 signal null
    at ChildProcess.<anonymous> (file:///home/yellowtent/box/src/shell.js:70:23)
    at ChildProcess.emit (node:events:508:28)
    at maybeClose (node:internal/child_process:1101:16)
    at ChildProcess._handle.onexit (node:internal/child_process:305:5) {
  reason: 'Shell Error',
  details: {},
  stdout: <Buffer >,
  stdoutString: '',
  stdoutLineCount: 0,
  stderr: <Buffer 63 70 3a 20 63 61 6e 6e 6f 74 20 73 74 61 74 20 27 73 6e 61 70 73 68 6f 74 2f 61 70 70 5f 36 64 61 38 35 63 39 61 2d 66 62 66 30 2d 34 35 36 33 2d 62 ... 45 more bytes>,
  stderrString: "cp: cannot stat 'snapshot/app_6da85c9a-fbf0-4563-baaa-1ad3080cf467': No such file or directory\n",
  stderrLineCount: 1,
  code: 1,
  signal: null,
  timedOut: false,
  terminated: false
}
2026-04-01T02:01:21.585Z storage/filesystem: SSH remote copy failed, trying sshfs copy
2026-04-01T02:01:21.585Z shell: filesystem: cp -aRl /mnt/managedbackups/99e49fad-652d-4a9b-ae14-2200e68188d8/snapshot/app_6da85c9a-fbf0-4563-baaa-1ad3080cf467 /mnt/managedbackups/99e49fad-652d-4a9b-ae14-2200e68188d8/2026-04-01-020004-530/app_next.mydomain.com_v5.7.0
2026-04-01T07:06:43.896Z backuptask: copy: copied successfully to 2026-04-01-020004-530/app_next.mydomain.com_v5.7.0. Took 18322.66 seconds

I've tried to setup the TrueNas share in multiple ways : with/out prefix ; directly on a (sub)dataset ; in a $HOME subfolder (i never used the $HOME only ) ; with/out 777 on the all folder/dataset dedicated to it....
I've made sure that it is not a performance bottleneck (on both side and in connection speed/bandwidth).
Every time it fail at the same point.

From what i understand of my reading, i guess it is a permission-related issue, but i have no idea in which way...
Any suggestion will be welcome .
Thanks in advance!

Additional technical details :

Cloudron version 9.1.5 running on Ubuntu 22.04.5 LTS ; hosted on "mydomain.com" vps 6GB ram 4 cores
Backup setup : sshfs + rsync + file encryption + hardlink enable ; file name encryption disable ; 
Data to backup : simple cloudron instance with "only" Nextcloud running ; 70 GB  125366 files
Backup target : TrueNas Scale 25.04 ; dedicated user ("myuser") ; dedicated dataset or directory. 

Ps : 98% percent of my integrity check fail, even on my primary target (Hetzner storage box) which don't have upper issue . But one issue after the other