be carefull of the ldap syntax.
the following string will not work :cn=User_Accounts,ou=Alfresco,dc=your_domain,dc=com
you must add backslash before the = example : cn\=User_Accounts,ou\=Alfresco,dc\=your_domain,dc\=com
To be sure to use the right ldap terms, go to your active directory users and computers snappin and click properties on the node you want to target your ldap search. now go on the attribute editor and look at attribue distinguishedName.
if it's an organisation unit the first term start with OU\= but not CN\=
Good luck.