Re: FortiClient breaks on MacOS Sierra !
kleptomac, the instructions given at your link are quite similar to those given above by Jpokki. As for me, I ended up running the uninstaller to start from scratch. Then, I downloaded 5.4.1 from the Fortinet site, then ran the setup to the point where the Install button appears. I then searched for the .dmg in the fctupdate folder, ran it from there and I'm good.