Not SquareEnix support, but I know their usual starting list for you is going to be:
  1. Try temporarily renaming your game's user data directory (usually somewhere under Documents in a 'My Games' directory) so that it creates a fresh one and see if that helps. VERY IMPORTANT NOTE: You want to rename it so you save the data. That way you can rename it back to put it back afterwards if needed. Since that game data holds all your gearsets and hotbar configurations, which are client-side, you do not want to lose them unless you've backed them up to the server already and so can restore them if needed.
  2. Try doing a clean boot of Windows to see if something that's running might be the problem.
  3. Try updating your graphics drivers, and/or clean reinstall of them.
  4. Try reinstalling the game.
  5. Try reinstalling the game specifically to drive C:

Based on what some others have seen in other threads relating to 6d, I'd add a few things to that list:
  1. Try toggling the launcher to the older mode (the gear icon on the launcher will bring up launcher settings).
  2. Try making certain that Windows UAC is enabled.
  3. Try temporarily making a new Windows user and running FFXIV as that user, to see if it's something in user-specific settings that's causing the issue. IMPORTANT NOTE: Remember that the hotbars and gearsets are in the client-side data; running FFXIV on a new user will mean you don't have those. If you use the secondary account for anything more than a quick test you'll either want to copy them over or pull them down from the server (if you already backed them up to the server).

Hopefully that helps... and if not, hopefully one of the SE support folks can get you up and running again.