It is not possible to use X-Forwarding into a stock FreeBSD machine, after you installed the xorg-server port. Even when /etc/ssh/sshd_config says that XForwarding is enabled. I tried this and did not get a DISPLAY variable.
And there was no error message at all. Searching the web did not reveal any results. The key to the solution was the ssh verbose mode. One line told about the xauth utility missing.
If you have a similar problem, then try to install x11/xauth from the ports. This should solve your problems.