Can confirm was able to start patch download, complete it then proceed to log into FFXIV without a problem when using a different windows user account.

Logging back into main/administrator account then booting up ffxiv launcher and trying to log in via that way caused this error to reappear, what could be the cause of something like this?

Anyways temporary fix for now, create or try using a different windows user account and booting up the ffxiv launcher from there instead. It should work.