Initialization failed with sftp backend

[varac]

I can't get resticker to work with a sftp backend (synonlogy NAS):

root@rock:~/resticker# docker-compose up
WARNING: The aW variable is not set. Defaulting to a blank string.
Starting resticker_backup_1 ... done
Attaching to resticker_backup_1
backup_1  | Checking configured repository 'sftp://xxx@abc.de/home/restic'...
backup_1  | Fatal: unable to open config file: Lstat: file does not exist
backup_1  | Is there a repository at the following location?
backup_1  | sftp://xxx@abc.de/home/restic
backup_1  | Could not access the configured repository. Trying to initialize (in case it has not been initialized yet) ...
backup_1  | Fatal: create repository at sftp://xxx@abc.de/home/restic failed: failed to send packet: write |1: file already closed
backup_1  | 
backup_1  | Initialization failed. Please see error messages above and check your configuration. Exiting.
resticker_backup_1 exited with code 1

My docker-compose.yml:

version: "3.3"

services:
  backup:
    image: mazzolino/restic
    hostname: docker
    environment:
      RUN_ON_STARTUP: "true"
      BACKUP_CRON: "0 30 3 * * *"
      RESTIC_REPOSITORY:  'sftp://xxx@abc.de/home/restic'
      RESTIC_PASSWORD: 'xxx'
      RESTIC_BACKUP_SOURCES: /mnt
      RESTIC_BACKUP_ARGS: >-
        --tag rock
        --exclude='.nodenv/versions'
        ...
        --verbose
      RESTIC_FORGET_ARGS: >-
        --keep-last 10
        --keep-daily 7
        --keep-weekly 5
        --keep-monthly 12
      TZ: Europe/Berlin
    volumes:
      - /:/mnt/:ro
      - ./known_hosts:/root/.ssh/known_hosts
      - /root/.ssh/borgbackup_key:/root/.ssh/id_rsa:ro

Any ideas how to fix/troubleshoot this ?

[varac]

Fyi, on the NAS I could see a restic directory structure beeing created.

[varac]

Also, changing the RESTIC_BACKUP_ARGS --verbose to --verbose=10 doesn't produce any more output. Might it be that the --verbose flag is completely ignored since it's applied after the backup subcmd, whereas restic expects the --verbose flag/option before the backup subcmd ?

[varac]

Nevermind, I realized I hit this restic bug with too old sftp servers.
Works well on a newer sftp server.