I am attaching a sample with a working example page of the intended behaviour. Generally the only difference to make it work is how you get the user. You need to use Sitefinity's UserManager instead of LdapMembershipProvider and just pass the Provider Name when you get the manager instance.
Here is the code snippet change that is relevant:
UserManager manager = UserManager.GetManager(
ldapId = UserName.Text;
User user = manager.GetUser(ldapId);
(user.IsApproved && !user.IsLockedOut)
"User was logged in, so signing out"
var validate = SecurityManager.AuthenticateUser(
LoginLabel.Text = validate.ToString();
Make sure your membership provider's name is actually "LdapUsers" and you should be good to go.
Let me know if this helps!
the Telerik team
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