To be exact, I get this message several times a week in my syslog server:

5  	auth  	auth  	error  	message forwarded from aixbox: sshd[283116]: error: subsystem: cannot stat /usr/libexec/sftp-server: no such file or directory

The message is coming from AIX 6.1 TL4 with openSSH 5.2.0.5300 installed. On another box with AIX 5.3 TL 6 and openSSH 5.0.0.5300 installed the error does not occur.

The problem is in the /etc/ssh/sshd_config file:

...<cut>...
# override default of no subsystems
Subsystem      sftp    /usr/libexec/sftp-server
...<cut>...

The mentioned sftp-server is not available, it should point to the sftp-server in /sbin:

# override default of no subsystems
#Subsystem      sftp    /usr/libexec/sftp-server
Subsystem      sftp    /usr/sbin/sftp-server

After this change, restart the ssh service:

SHooft@aixbox:/home/SHooft>sudo /etc/rc.d/rc2.d/Ssshd stop
0513-044 The sshd Subsystem was requested to stop.
SHooft@aixbox:/home/SHooft>sudo /etc/rc.d/rc2.d/Ssshd start
0513-059 The sshd Subsystem has been started. Subsystem PID is 143726.

Now the message is gone.