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.
From PC-BSD Wiki
- attach a copy of your saved /tmp/pc-sysinstall.log; if you did not save it during the installation, a copy was saved for you to /root/pc-sysinstall.log