Sorry for the inconvenience. Known Navigation header breakage.
There may be other unforeseen breakage primarily in the handbook portion of the wiki.
The Table Of Contents section of each "PC-BSD® Users Handbook" page may be the best workaround.
This will drop you down to a console where you can try the instructions in theto manually configure and test Xorg. Once you have a configuration that works for you, save it to /etc/X11/xorg.conf, and restart gdm to test the configuration:
<ref>tags exist, but no
<references/>tag was found