ActiveDirectoryMembershipProvider do not work ?

[zxc]

Hello,

I cannot start using Forms Authentication with Active Directory. On call of:

Membership.ValidateUser(StdFormItemLogin.Text, StdFormItemPassword.Text)

I get a ConfigurationErrorsException:

Object reference not set to an instance of an object. (C:\...\web.config
line 33)

My web.config is (fragment):

28: <membership defaultProvider="ps">
29: <providers>
30: <add name="ps"
31: connectionStringName="psCS"
32: attributeMapUsername="samAccountName"
33: type="System.Web.Security.ActiveDirectoryMembershipProvider"
34: />
35: </providers>
36: </membership>

LDAP connection string looks OK ...

I've try many combinations but cannot get more detailed error info. It looks
that ActiveDirectoryMembershipProvider is not created for some reason but
why ... ?

Any ideas are welcome.
zxc