Avm / fritzbox configuration for rclone / restic

Fritzbox: this is a wlan router by AVM. I cannot really configure too much on it. FTP in principle works. It seems that ftp is not understanding all commands. Some work just fine:

$ restic -r rclone:fritzbox:TEST snapshots
enter password for repository:
repository 8436a127 opened successfully, password is correct
ID Time Host Tags Paths

7790eae7 2019-08-11 09:03:03 lapin18 /tmp/A
0180fe57 2019-08-11 09:05:44 lapin18 /etc/fstab

I can also recover e.g. /etc/fstab from the backup.
But:

$ rclone ls fritzbox:
2019/08/12 16:45:35 ERROR : : error listing: 500 'LIST ': command not understood.
2019/08/12 16:45:35 Failed to ls with 2 errors: last error was: 500 'LIST ': command not understood.
(paulus@lapin18)-(~)-(16:45:35): rclone -vv ls fritzbox:
2019/08/12 16:45:39 DEBUG : rclone: Version "v1.48.0" starting with parameters ["rclone" "-vv" "ls" "fritzbox:"]
2019/08/12 16:45:39 DEBUG : Using config file from "/home/paulus/.rclone.conf"
2019/08/12 16:45:39 DEBUG : ftp://fritz.box:21: Connecting to FTP server
2019/08/12 16:45:40 ERROR : : error listing: 500 'LIST ': command not understood.
2019/08/12 16:45:40 Failed to ls with 2 errors: last error was: 500 'LIST ': command not understood.

... to be continued (as I get the message that my post has too many links)

Dietrich