Using AIX 6.1 boxes.
User user1 connects from box A to box B using ssh.
When password authentication is used everything is fine. When I configure user1 to use public key authentication sftp client works fine(no password asked), but ssh client fails. This is sshd log:
Accepted publickey for user1 from A port 46607 ssh2
Disconnecting: Remote login for account user1 is not allowed.
To my understanding ssh authentication passes fine - something else in the way. I've done identical configs on linux boxes with no problems.
This is something AIX specific.
If your user is only going to be using SFTP and not require access to your box via SSH, you can set their "shell" to "/usr/sbin/sftp-server". The /usr/sbin/sftp-server is part of the SSH fileset included with AIX I believe. I've done this for quite a few users who will never login via SSH, they are just allowed to upload code. It even supports SSH key authentication. It isn't a big deal but it makes your box better secured.
Also, if you do decide to use it, make sure "/usr/sbin/sftp-server" is included on the "shells = ..." line in your /etc/security/login.cfg file, otherwise, it won't work.