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 operating system upgrade, read the caveats listed in Update manager. Be sure to backup your important data to another system or external device before starting the upgrade.