Thank you for the reply and I apologize for the delay in our response.

Why exactly do you think that Windows Defender isn't the cause? Have you already tried disabling it?

You can also try to change the settings in Internet Explorer:

Ensure that the latest version of Internet Explorer is installed on your PC (even if you don't use Internet Explorer), please do not use Edge as that is a different internet platform. Once Internet Explorer is up-to-date, please open the IE browser, open the "Tools", go to "Internet Options" and adjust the following settings:
- Under the "General" tab, please delete ALL browser history.
- Under the "Security" tab, please move the slider to the lowest possible settings or no higher than "Medium"
- Under the "Privacy" tab, please move the slider to the lowest possible settings or to "Allow All Cookies"
- Under the "Advanced" tab, please make sure that "Allow software to run or install even if signature is invalid" is CHECKED.
- Make sure that all SSL and TLS (EXCEPT SSL 3.0) are CHECKED. [SSL 3.0 should be UNCHECKED]
- Hit "Apply" to save the settings, and then "OK, to close the Internet Options menu.
- Right-Click on the FFXIV launcher icon and select "Properties"
- Go to the "Compatibility" tab, and change the compatibility mode to "Windows 7"
- Hit "Apply", then "OK" to save the changes.
- Please RESTART your PC now. Once your PC has been restarted, Right-Click the FFXIV launcher icon once more and select "Run as Administrator"

We hope we can be of assistance in helping you enjoy the exciting world of Final Fantasy XIV!