My workaround for the treacherous 401/406 error is buying the Ultimate Game Card and using that to pay for crysta. Then use that Crystal to purchase subscription of FF14. It's a tedious process but this seems to work for me every time. I also get 401/406 error when I try to use my credit card to purchase Crysta directly from the Square Enix Account Management site.

So...

1. Purchase Ultimate Game Card from pcgamesupply (You get the codes immediately)
2. Purchase Crysta using Ultimate Game Card from Square Enix Account Management Site
3. Log into Mog Station and purchase subscription using Crysta