Remove the old installation completely and then redownload from App Store. Then re-register and you should be getting in. Be mindful though, there are some bugs with class selection; you might get different class from what you choose.
Based on their FB post, class selection bug should be fixed tomorrow so I'm waiting for that before going any further after registering new account.
The game is full of buggs.. these happens to me a few times every day.
Battles just ending with out reason.
Brewing puzzle gets skipped, instantly giving a weak potion.
Cauldrons showing completed potions, but when tapped says the potions is already goted (yeah it says goted).
Ingredients don't disappear from the map when collected.
Different parts of the tutorial texts randomly pops up, even though I completed the tutorial.
Some buffs and debuffs have no effect.
Don't get me wrong, I love the game and I want it to succeed.. But these are quite annoying, but it is alpha, so they will be fixed, sooner or later. :)