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There's a 5 page posting related to login problems, and that also links to a couple of other postings where people have asked the same question.
Read it
HEREAlthough it's a biggy, I suggest you read ALL of it, because chances are that one of the postings in that thread will give you the solution. In my case (post 34 from memory I think) it was the MySQL table that tracks your logins that got corrupted and needed flushing. But it could be firewalls, cokkie settings and much more. Read the thread and good luck.
Ted