Thank you for your answer. Did the following (via webinterface)
- flashed the new image
- mounted the ssd
- created a user (sftpuser - sftp-access, users)
- created a share on the ssd (chroot - drwxrwsrwx+ 2 root root 4.0K Sep 13 05:13 chroot) - it is listed in /sharedfolders
- checked the access list (sftp-setting) and everything is ok/enabled - AllowGroups is disabled
But when I try to sftp now, I get the error:
debug1: Authentication succeeded (password).
Authenticated to 192.168.1.16 ([192.168.1.16]:222).
debug2: fd 6 setting O_NONBLOCK
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: pledge: network
packet_write_wait: Connection to 192.168.1.16 port 222: Broken pipe