If Sql Server database collation is Turkish_CI_AS, I get that error at below cause UPPER function in SELECT Query in "GetUserId" method of SimpleMembershipProvider.cs class.
SQL UPPER('info') is not equal to "info".ToUpperInvariant() in Turkish_CI_AS
Sql Server Database Collation : Turkish_CI_AS
WebSecurity.CreateUserAndAccount("info", "123456");
Exception Details: System.Web.Security.MembershipCreateUserException: The Provider encountered an unknown error.