Sorry for the inconvenience. Known Navigation header breakage, its being "rebuilt."
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.
- before performing any upgrade, always back up your important data to an external backup device, such as a removable USB drive, or to another system using a utility such as Life Preserver. While it is unlikely that something will go wrong, you will be glad that you made that backup just in case.