I had same issue after the patch so i tried literally everything and stumbled upon your post and did some workaround about
"The only thing that has worked so far is to create an entirely new Windows user and running FFXIV from this new user account. However, this is an extremely suboptimal solution as it essentially means having to painstakingly recreate my primary user account settings. I might as well wipe my computer and start over at that point.".
So i found out that if you remove your windows account from administrator group then error disappears.