Never heared from SSH Tectia. Just was at their site...
Maybe it is no big help, but why not use the ssh/sshd that comes with AIX 5.3 for free???

Else you might check if the sshd_config or if something similar exists with that kind of sshd contains something awkward compared to those of the boxes that work.
Does this Tectia sshd write some log? Something in your errpt?