Huh! I'm glad that worked for you. I couldn't say offhand why UAC being disabled would break the latest build, but it is known to break some things. I mean, you shouldn't have UAC disabled in the first place—as you no doubt know given your 'no idea how it got disabled' remark—but turning it off entirely can cause Explorer previews to die, among other things. Despite being tech-savvy, I could not offhand tell you why on earth that is, but it's certainly possible that all the changes to FFXIV's executable for 5.2 have made it susceptible to that sort of issue.

¯\_(ツ)_/¯