Hello, i had this problem recently and after an extensive 2 days internet research i saw that many people had the same problem at various times (Usually after a patch or after trying to install the game for the first time). The only workaround i found is to create another account on windows 10 and try to run the game from this account, which worked fine.

Although even after patching if i go back to my main windows 10 account i still get the same error and not being able to open the game from there. So i see this as a temporary solution mainly for two reasons:

1) Some people in threads that suggested this solution (making a second account) said that they had the same problem again when a new patch released and had to create yet another account. Obviously this is not a permanent solution if every time you have to patch the game you have to create an extra account.
2)I think its silly to have a second account only to be able to play FFXIV.

All other troubleshooting like editing the cfg file in documents>FFXIV, changing DNS, clean boot, disabling firewall, disabling User Account control, changing the internet explorer settings etc have not fixed the issue.

Has anyone figured out what is causing the problem on the main w10 account and why its getting fixed by just creating a second one and running the game from there?

Obviously there is a difference between the two accounts that lets the game run, but i can think of anything else to try.