I just got hit with this when logging in just now, and I tried the earlier tricks of backing up your game folder, then re-installing the client, then changing the game folder back, but it didn't work.

Then I tried opening the ffxivgame.ver file in the {install}/game folder, where it had "2017.08.23.0000.0000" as the value. I changed it to something slightly lower, "2017.08.22.0000.0000", then restarted the boot client and logged in. It downloaded the old 10 MB patch again from last week, and when it launched the game, it finally let me in.

Hopefully this works for anyone else.