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.
Once the downloads are finished, a pop-up message will indicate that the system is now ready to reboot in order to finish the upgrade. PC-BSD® will not automatically reboot the system, giving you the opportunity to close any applications that you have open before you reboot the system yourself.