So from what I've been able to see across these threads (and folks in my FC having the 6d problem), there's three possible causes/solutions.
  1. Issue with the launcher permissions. In this case, swapping the launcher to use the old-style launcher (which can be done from the launcher's settings menu) will fix things for people.
  2. Issue with computer erroneously not using discrete graphics. Most common on gaming laptops. In this case, you can fix it by launching from GeForce Experience (to force things into High Performance mode) or by tweaking the game executable's settings in the NVidia display control panel.
  3. Issue with security APIs when Windows UAC is disabled. For folks who've turned off Windows UAC (the screen that pops up when something wants to do a secure/administrator operation), it seems some API coughs up a hairball when FFXIV tries to get permissions it wants. The fixes here are to either a) re-enable UAC, or b) make a new account with UAC still enabled and play on that account.

Since you mentioned making a new Windows account fixed things, I'm guessing #3 is the one biting you.