It is a client side issue or possibly an issue with your internet.

While it would be a lot of work to try, have you tried to reinstall your computer from scratch?
That way you certainly won't have any old beta files that may interfere in some way.

I do assume that you've really uninstalled all previous FFXIV instances and cleaned your My Documents folder and checked in Program Files for any SE/FFXIV folders left after uninstalling.