What is about these following parameters in sshd_config file ?
meanwhile I figured out what is causing the issue:
the /X11/session/xauth package was not installed,
although the installation is called solaris-large-server.
The x11 packages are all considered as non-essential for a server configuration. They are all installed as part of the solaris-desktop group package, which can be applied on top of a server installation when a graphical interface is required.