Unfortunately you need to be using LDAPS (otherwise Active Directory doesn't let you change any value, much less passwords), and the Moodle bind user needs to have the privilege to change other users' passwords.
This last requirement was a limitation of the way PHP LDAP module did some operations. Starting with PHP 5.6.0 they have implemented ldap_modify_batch (which I have just seen it's been backported to PHP 5.4.26 to 5.5.10). This new feature allows[1] for the users to change their own passwords, without needing special privileges for the bind user. But you would still need LDAPS.
[1] Of course, all the code needed to implement this in Moodle has to be written, but the missing PHP functionality is at least there.
Saludos.
Iñaki.
Authentication
LDAP Error
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