Error when changing password in AD LDS

Question

I have problem when implemented password updating of user in AD LDS: it throws below error when calling connection.Modify(). Does anybody have experience with ADLDS and its errors?

Operations Error:00002077: SvcErr: DSID-03380736, problem 5012 (DIR_ERROR), data 8237

Code as below:

        var entry = GetUserEntry(userName, AttributeList(AttributeTypes.Basic));
        //create the ldap modifications
        var modifications = new LdapModification[2];
        var deletePassword = new LdapAttribute(application.UserPasswordAttribute, oldPassword);
        modifications[0] = new LdapModification(LdapModification.DELETE, deletePassword);            

        var addPassword = new LdapAttribute(application.UserPasswordAttribute, newPassword);
        modifications[1] = new LdapModification(LdapModification.ADD, addPassword);

        //perform the modification
        connection.Modify(entry.DN, modifications);

The entry.DN includes: "CN=user1,CN=Users,CN=sampleInstance,DC=local,DC=com". By the way, I'm using SSL connection.