Invalid character in a Base-64 stringInvalid character in a Base-64 string. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it

Hello,
my website was built with VS2008, I used MS Membership to handle login . Now, when I login, I get this error
Invalid character in a Base-64 string. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.FormatException: Invalid character in a Base-64 string. Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [FormatException: Invalid character in a Base-64 string.] System.Convert.FromBase64String(String s) +0 System.Web.Security.MembershipProvider.EncodePassword(String pass, Int32 passwordFormat, String salt) +54 System.Web.Security.SqlMembershipPro ...