When using Sun Secure Global Desktop, it is not possible to log to another machine, and start an X11 application to the original display. The connection fails, with no apparent reason.
The solution is to connect to the remote system using ssh with the -X option. This will setup a "dummy" DISPLAY, which will work.
gotcha: if the remote system is Solaris 10 (or Open Solaris), and the remote user is root, ssh is disabled for root. When trying to connect, it reacts as if the password was wrong.
The solution is to edit "/etc/ssh/sshd_config", and change the "PermitRootLogin" line. Then restart the ssh service:
# svcadm restart svc:/network/ssh:default