It is recommended to install the game on the C drive with the OS, which you seem to have done to fix the issue.

Though, I wouldn't repair, I would uninstall the game completely first, and then re-download it from scratch.

But, the easiest way to fix corrupted installs is, after you download and update the client anew or for the first time, go to the FFXIV folder (/SquareEnix/Final Fantasy XIV - A Relam Reborn/) and copy it to an external drive or to another folder on one of your drives, so then if you get an issue like this (above) you can simply copy and paste to overwrite the old install that got corrupted, I do this myself when I reinstall my OS to refresh my 60 day antivirus subscription lol... saves paying for it! and re-downloading 68gb isn't fun.