I have this same issue. I have been walked through everything under the sun to fix it. I can sign in with my mothers account no worries, but mine fails - yet it is somehow a mystery to the support staff. Yet, online it is quite a common concern and problem. The support people are quite nice and I recommend calling them to get your Apple ID on a list of whenever fixes at this point. It is rather frustrating that they do not have a fix in place to make this go away, but this is why people are extremely frustrated with Apple. Fixes are very slow to come around. If you do look online... you will see this has been a common problem before iOS 6.