I was having the same issue. Could log into an alt on Malboro, but not my main on Balmung. Read somewhere that devs were aware of it, and that it was caused by ports and congestion or something.
So a thought popped into my head, and i used a program i have from portforward.com to change my static IP by just 1 digit ( ___.__.__.158 - ___.__.__.159) and.. ta-da! Can play my main again. Not sure how long it will last, but it's the first I've even been able to MOVE on her in over 24 hours without getting INSTANTLY 90000'd and kicked.