I have set up an sftp/scp only shell for a jailed user (guest, pwd guest) under Raspbian Stretch and following very precisely the steps in the jailkit howto instructions for that scenario, except for the change in user name.
On sftp address@hidden the connection closes immediately after entering the password. I enclose a copy of the relevant lines in auth.log. The key issue appears to be that jk_lsh cannot find info for guest at line 7; this is followed by a disconnect from "me", i.e. user pi, ::1.
The two passwd files appear to be in order, see enclosed copies where passwd is /etc/passwd and jail_passwd is /srv/sftpjail/etc/passwd.
This is driving me a bit frantic so any help much appreciated!