Login is case-sensitive, so make sure you are entering your exact avatar name.
Note that you should not be entering your display name or Second Life username, but your "legacy" avatar name. For example, if your avatar has a first and last name (John Smith for example), you would enter your avatar name as John Smith. If you have a newer account without a last name, add Resident to your avatar name - you would log in as john Resident for example.
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If you are still unable to log in, please contact us by opening a support ticket and we'll look into it.