Membership.ValidateUser asp.net 4.5

I've a webapp (webform) compiled in .NET 3.5 that use thi code in login:
Membership.ValidateUser(txtUserName.Text, txtPassword.Text)
If modify .NET 3.5 with 4.0 or 4.5 Membership.ValidateUser not works.
Why?
Same user and some computer..
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