The problem is that we use FormsAuthenticationTicket.UserData property to store additional user information which is specific for Sitefinity. In your case FormsAuthenticationTicket.UserData is an empty array and this results in the error you see. We made some changes to the code which provides some more descriptive information about the error.
Is there any reason for not using our built-in Login controls which will not cause such a issue? I tried to replicate the issue by using the standard ASP.NET Login control with no avail. Probably there is something in the logic of your custom control that sends data to FormsAuthenticationTicket.UserData and then we read it in an inappropriately.
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