@Brenden Kehren: You're right. I have just checked the validation of the user but not logging in. I changed the codes and it's working now.
UserInfo ui = MembershipContext.AuthenticatedUser;
UserInfo ui = UserInfoProvider.GetUserInfo(UserName.Text.Trim());
@Joshua Adams: Yes, my control is cloned from Login Form Webpart. I want to show a modal instead of redirect/refresh page as the Login Webpart do after logging in. So, I have replace the ASP Login control by my custom login form, but still keeping useful methods.